DEV: Call `MessageBus.stop()` after each system test (#21848)

## What is the problem?

MessageBus by default uses long polling which keeps a connection
open for 25 seconds by default. The problem here is that Capybara does not know about these
connections being kept opened by MessageBus and hence does not know how
to stop these connections at the end of each test. As a result, the long polling MessageBus connections are kept opened by the browser and we hit chrome's limit of 6 concurrent requests per host, new request made in the browser is marked as "pending" until a request is freed up. Since we keep a MessageBus long polling connection opened for 25 seconds, our finders in Capybara end up hitting Capybara's wait time out causing the tests to fail.  

## What is the fix?

Since we can't rely on Capybara to close all the existing Capybara
connections, we manually execute a script to stop all MessageBus
connections after each system test.

```
for i in {1..10}; do
  echo "Running iteration $i"
  PARALLEL_TEST_PROCESSORS=8 CAPYBARA_DEFAULT_MAX_WAIT_TIME=10 bin/turbo_rspec --seed=34908 --profile --verbose --format documentation spec/system
  if [ $? -ne 0 ]; then
    echo "Error encountered on iteration $i"
    exit 1
  fi
done
echo "All 10 iterations completed successfully"
```

Without the fix, the script fails consistently in the first few iterations. Running in non-headless mode with the "network" tab opened will reveal the requests that are marked as pending.

spec/rails_helper.rb

@@ -442,6 +442,7 @@ RSpec.configure do |config|
       end
     end
 
+    page.execute_script("if (typeof MessageBus !== 'undefined') { MessageBus.stop(); }")
     Capybara.reset_sessions!
     Capybara.use_default_driver
     Discourse.redis.flushdb

spec/system/admin_customize_form_templates_spec.rb

@@ -9,10 +9,6 @@ describe "Admin Customize Form Templates", type: :system, js: true do
   fab!(:category) { Fabricate(:category) }
 
   before do
-    skip(<<~TEXT) if ENV["CI"]
-    The specs here are extremely flaky on CI for some reason.
-    TEXT
-
     SiteSetting.experimental_form_templates = true
     sign_in(admin)
   end
@@ -21,25 +17,29 @@ describe "Admin Customize Form Templates", type: :system, js: true do
     before { category.update!(form_template_ids: [form_template.id]) }
 
     it "should show the existing form templates in a table" do
-      visit("/admin/customize/form-templates")
+      form_template_page.visit
+
       expect(form_template_page).to have_form_template_table
       expect(form_template_page).to have_form_template(form_template.name)
     end
 
     it "should show the categories the form template is used in" do
-      visit("/admin/customize/form-templates")
+      form_template_page.visit
+
       expect(form_template_page).to have_form_template_table
       expect(form_template_page).to have_category_in_template_row(category.name)
     end
 
     it "should show the form template structure in a modal" do
-      visit("/admin/customize/form-templates")
+      form_template_page.visit
+
       form_template_page.click_view_form_template
       expect(form_template_page).to have_template_structure("- type: input")
     end
 
     it "should show a preview of the template in a modal when toggling the preview" do
-      visit("/admin/customize/form-templates")
+      form_template_page.visit
+
       form_template_page.click_view_form_template
       form_template_page.click_toggle_preview
       expect(form_template_page).to have_input_field("input")
@@ -55,7 +55,7 @@ describe "Admin Customize Form Templates", type: :system, js: true do
   end
 
   def quick_insertion_test(field_type, content)
-    visit("/admin/customize/form-templates/new")
+    form_template_page.visit_new
     form_template_page.type_in_template_name("New Template")
     form_template_page.click_quick_insert(field_type)
     expect(ace_editor).to have_text(content)
@@ -63,7 +63,7 @@ describe "Admin Customize Form Templates", type: :system, js: true do
 
   describe "when visiting the page to create a new form template" do
     it "should allow admin to create a new form template" do
-      visit("/admin/customize/form-templates/new")
+      form_template_page.visit_new
 
       sample_name = "My First Template"
       sample_template = "- type: input"
@@ -75,40 +75,41 @@ describe "Admin Customize Form Templates", type: :system, js: true do
     end
 
     it "should disable the save button until form is filled out" do
-      visit("/admin/customize/form-templates/new")
-      expect(form_template_page).to have_save_button_with_state(true)
+      form_template_page.visit_new
+      expect(form_template_page).to have_save_button_with_state(disabled: true)
       form_template_page.type_in_template_name("New Template")
-      expect(form_template_page).to have_save_button_with_state(true)
+      expect(form_template_page).to have_save_button_with_state(disabled: true)
       ace_editor.type_input("- type: input")
-      expect(form_template_page).to have_save_button_with_state(false)
+      expect(form_template_page).to have_save_button_with_state(disabled: false)
     end
 
     it "should disable the preview button until form is filled out" do
-      visit("/admin/customize/form-templates/new")
-      expect(form_template_page).to have_preview_button_with_state(true)
+      form_template_page.visit_new
+      expect(form_template_page).to have_preview_button_with_state(disabled: true)
       form_template_page.type_in_template_name("New Template")
-      expect(form_template_page).to have_preview_button_with_state(true)
+      expect(form_template_page).to have_preview_button_with_state(disabled: true)
       ace_editor.type_input("- type: input")
-      expect(form_template_page).to have_preview_button_with_state(false)
+      expect(form_template_page).to have_preview_button_with_state(disabled: false)
     end
 
     it "should show validation options in a modal when clicking the validations button" do
-      visit("/admin/customize/form-templates/new")
+      form_template_page.visit_new
       form_template_page.click_validations_button
       expect(form_template_page).to have_validations_modal
     end
 
     it "should show a preview of the template in a modal when clicking the preview button" do
-      visit("/admin/customize/form-templates/new")
+      form_template_page.visit_new
       form_template_page.type_in_template_name("New Template")
       ace_editor.type_input("- type: input")
       form_template_page.click_preview_button
+
       expect(form_template_page).to have_preview_modal
       expect(form_template_page).to have_input_field("input")
     end
 
     it "should render all the input field types in the preview" do
-      visit("/admin/customize/form-templates/new")
+      form_template_page.visit_new
       form_template_page.type_in_template_name("New Template")
       ace_editor.type_input(
         "- type: input\n- type: textarea\n- type: checkbox\n- type: dropdown\n- type: upload\n- type: multi-select",

spec/system/page_objects/pages/form_template.rb

@@ -3,6 +3,16 @@
 module PageObjects
   module Pages
     class FormTemplate < PageObjects::Pages::Base
+      def visit
+        page.visit("/admin/customize/form-templates")
+        self
+      end
+
+      def visit_new
+        page.visit("/admin/customize/form-templates/new")
+        self
+      end
+
       # Form Template Index
       def has_form_template_table?
         page.has_selector?("table.form-templates__table")
@@ -67,12 +77,12 @@ module PageObjects
         find(".form-templates__form-name-input").value == name
       end
 
-      def has_save_button_with_state?(state)
-        find_button("Save", disabled: state)
+      def has_save_button_with_state?(disabled:)
+        find_button("Save", disabled:)
       end
 
-      def has_preview_button_with_state?(state)
-        find_button("Preview", disabled: state)
+      def has_preview_button_with_state?(disabled:)
+        find_button("Preview", disabled:)
       end
     end
   end