Fix Qunit integration

[steveszc]

Qunit no longer supports adding a test after the test run has finished. This was apparently never intentionally supported behavior in Qunit but we happened to be relying on it to inject the memory-leak-detection test. A recent Qunit PR removed support for this behavior and memory leak detection no longer runs reliably in Qunit 2.17+, reported in #46 and #47. In newer versions of Qunit this logs a warning, but in Qunit v3 it will hard error.

I've raised this with Qunit team here and they have created an issue to track a new path forward for plugins to run a "final" test. Fortunately they seem to agree that there should be a clear way for plugins to do this.

Once Qunit has introduced a new way forward, we need to update the way we run the final memory leak detection test.

[Krinkle]

@steveszc Just checking in to see if you've had a chance to consider my suggestion at qunitjs/qunit#1663 (comment).

My understanding is that you currently use Testem.afterTests at

ember-cli-memory-leak-detector/packages/ember-cli-memory-leak-detector/lib/templates/test-body-footer.html

Lines 26 to 27 in 7bbbb32

	if (typeof QUnit !== "undefined") {
	Testem.afterTests(detectMemoryLeaksWithQUnit);

Which in turn is fired by Testem.on('all-test-results') at https://github.com/testem/testem/blob/bba1a9c64a045767754f1de77144bbbe388f43d6/public/testem/testem_client.js#L349, and ultimately from QUnit.done() at https://github.com/testem/testem/blob/49853346b4b8945e983671be155c7ae062376f4a/public/testem/qunit_adapter.js#L114-L116.

Using QUnit.begin() here instead of Testem.afterTests() might do the trick? There's already a conditional for QUnit. If you prefer a Testem abstraction, a one-off listener for tests-start might work. It looks like the Jasmine adapter emits this already once without data, to signal the start of the run, just like QUnit.on('runStart') and QUnit.begin(). Perhaps Testem would be interested in providing a hook like Testem.beforeTests that would work based on a once Testem.on('tests-start') listener or some such. But - for now, I'm hoping QUnit.begin() will work for you?