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/helpers/application_helper.rb

@@ -13,9 +13,11 @@ module ApplicationHelper
     @extra_body_classes ||= Set.new
   end
 
-  def discourse_config_environment
+  def discourse_config_environment(testing: false)
+
     # TODO: Can this come from Ember CLI somehow?
-    { modulePrefix: "discourse",
+    config = {
+      modulePrefix: "discourse",
       environment: Rails.env,
       rootURL: Discourse.base_path,
       locationType: "auto",
@@ -32,7 +34,16 @@ module ApplicationHelper
         version: "#{Discourse::VERSION::STRING} #{Discourse.git_version}",
         exportApplicationGlobal: true
       }
-    }.to_json
+    }
+
+    if testing
+      config[:environment] = "test"
+      config[:locationType] = "none"
+      config[:APP][:autoboot] = false
+      config[:APP][:rootElement] = '#ember-testing'
+    end
+
+    config.to_json
   end
 
   def google_universal_analytics_json(ua_domain_name = nil)