FEATURE: Allow theme tests to be run in production (#12815)

This commit allows site admins to run theme tests in production via a new `/theme-qunit` route. When you visit `/theme-qunit`, you'll see a list of the themes/components installed on your site that have tests, and from there you can select a theme or component that you run its tests.

We also have a new rake task `themes:install_and_test` that can be used to install a list of themes/components on a temporary database and run the tests of the themes/components that are installed. This rake task can be useful when upgrading/deploying a Discourse instance to make sure that the installed themes/components are compatible with the new Discourse version being deployed, and if the tests fail you can abort the build/deploy process so you don't end up with a broken site.

lib/tasks/javascript.rake

@@ -185,6 +185,15 @@ def dependencies
       source: 'route-recognizer/dist/route-recognizer.js.map',
       public_root: true
     },
+    {
+      source: 'qunit/qunit/qunit.js'
+    },
+    {
+      source: 'pretender/pretender.js'
+    },
+    {
+      source: 'sinon/pkg/sinon.js'
+    },
 
   ]
 end