INT-3297: Refactor tests to use BDD style (tinymce#516)

* INT-3297: Refactored helpers to not use any agar chaining

* INT-3297: Refactored `Loader.tsx` to not use any agar chaining and simplified context usage

* INT-3297: Refactored tests to use newer style BDD testing

* INT-3297: Fix linting issues

* INT-3297: Added `Loader.withVersion` for using miniature to preload TinyMCE

Also removed `deleteTinyMCE` from Loader.tsx and moved it to
LoadTinyTest.ts