[bug] Regression - Cross-Origin Read Blocking error when attempting to run tests

[mike-north]

To reproduce this problem

• go here: https://canary.ember-twiddle.com/a06ae7420d8d18e5907f45768237d1fb
• click the "run tests" checkbox

You'll see that the test UI is not shown. This twiddle worked fine for months -- I started to get reports on July 27 that this issue blocked people from seeing the QUnit UI.

[Gaurav0]

Here's the fixed version:

https://canary.ember-twiddle.com/410425e1676919ced96e7370d4dcdd4d?openFiles=tests.test-helper.js%2C

This is the cause:

#616