REFACTOR: Use imports for sinon and setResolver

I also took the opportunity with this commit to move some test specific
stuff out of `discourse-loader` which is loaded on the front end of the
application. The test module building now happens in the `test_helper`
bundle.

plugins/discourse-local-dates/test/javascripts/lib/local-date-builder-test.js.es6

@@ -1,5 +1,6 @@
 import I18n from "I18n";
 import LocalDateBuilder from "./local-date-builder";
+import sinon from "sinon";
 
 const UTC = "Etc/UTC";
 const SYDNEY = "Australia/Sydney";