Convert QUnit tests to mocha

[Jaifroid]

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Since we already use mocha for assertion in e2e tests, it would make sense to use it in Unit tests also. We currently run Unit tests in UI-based framework (QUnit) that runs directly in browsers, and as a result we have to run them in a browser-testing suite (testcafe) instead of directly in a Node framework like mocha. This adds overhead and is counterintuitive. It is also confusing, because there seems to be some blurring of lines between Unit and e2e testing.

I think it's time to drop QUnit.