DEV: Support for running theme test with Ember CLI

This is quite complex as it means that in production we have to build
Ember CLI test files and allow them to be used by our Rails application.

There is a fair bit of glue we can remove in the future once we move to
Ember CLI completely.

app/controllers/bootstrap_controller.rb

@@ -69,7 +69,7 @@ class BootstrapController < ApplicationController
       locale_script: locale,
       stylesheets: @stylesheets,
       plugin_js: plugin_js,
-      plugin_test_js: [script_asset_path("plugin_tests")],
+      plugin_test_js: [script_asset_path("plugin-tests")],
       setup_data: client_side_setup_data,
       preloaded: @preloaded,
       html: create_html,

app/controllers/qunit_controller.rb

@@ -8,15 +8,28 @@ class QunitController < ApplicationController
   }
   layout false
 
+  def is_ember_cli_proxy?
+    request.headers["HTTP_X_DISCOURSE_EMBER_CLI"] == "true"
+  end
+
   # only used in test / dev
   def index
-    raise Discourse::NotFound.new if request.headers["HTTP_X_DISCOURSE_EMBER_CLI"] == "true"
+    raise Discourse::NotFound.new if is_ember_cli_proxy?
     raise Discourse::InvalidAccess.new if Rails.env.production?
   end
 
   def theme
     raise Discourse::NotFound.new if !can_see_theme_qunit?
 
+    @is_proxied = is_ember_cli_proxy?
+    @legacy_ember = Rails.env.development? && !@is_proxied
+
+    # In production mode all bundles use `application`
+    @app_bundle = "application"
+    if Rails.env.development? && @is_proxied
+      @app_bundle = "discourse"
+    end
+
     param_key = nil
     @suggested_themes = nil
     if (id = get_param(:id)).present?