DEV: Support for running theme test with Ember CLI (second attempt)

This PR includes support for running theme tests in legacy ember
production envrionments.

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 +8,32 @@ 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 = if Rails.env.production?
+      ENV['EMBER_CLI_PROD_ASSETS'] != "1"
+    else
+      !@is_proxied
+    end
+
+    # 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?