DEV: Enable concurrent system tests (#19632)

Currently the `turbo:spec` task will fail when encountering system
tests as Capypara tries to use the same port for each process.

This simple change uses the same strategy as for databases, by just
incrementing the port number by `TEST_ENV_NUMBER` for each process.

spec/rails_helper.rb

@@ -249,7 +249,7 @@ RSpec.configure do |config|
 
     Capybara.configure do |capybara_config|
       capybara_config.server_host = "localhost"
-      capybara_config.server_port = 31337
+      capybara_config.server_port = 31337 + ENV['TEST_ENV_NUMBER'].to_i
     end
 
     chrome_browser_options = Selenium::WebDriver::Chrome::Options.new(