FIX: Cleanup DiscoursePluginRegistry state after tests that use it

This was causing some heisentests

spec/components/stylesheet/compiler_spec.rb

@@ -40,6 +40,7 @@ describe Stylesheet::Compiler do
       after do
         Discourse.plugins.pop
         Stylesheet::Importer.register_imports!
+        DiscoursePluginRegistry.reset!
       end
 
       it "does not include theme variables in plugins" do

spec/components/stylesheet/manager_spec.rb

@@ -68,7 +68,7 @@ describe Stylesheet::Manager do
 
   describe 'digest' do
     after do
-      DiscoursePluginRegistry.stylesheets.delete "fake_file"
+      DiscoursePluginRegistry.reset!
     end
 
     it 'can correctly account for plugins in digest' do

spec/lib/content_security_policy_spec.rb

@@ -4,6 +4,10 @@ require 'rails_helper'
 describe ContentSecurityPolicy do
   before { ContentSecurityPolicy.base_url = nil }
 
+  after do
+    DiscoursePluginRegistry.reset!
+  end
+
   describe 'report-uri' do
     it 'is enabled by SiteSetting' do
       SiteSetting.content_security_policy_collect_reports = true