HTML Reporter: Move QUnit.reset back to core to run it before testDone

Since the HTML reporter will always be the first to register the testDone callback, there's no way to do custom fixture cleanup before QUnit.reset runs. Moving this in src/test.js makes sure that the fixture reset runs after any custom testDone callbacks, allowing proper cleanup.

This is still only an intermediate solution, since we can't just remove QUnit.reset without addressing the need for custom resets. The source comment in front of the QUnit.reset call is therefore somewhat misleading. I guess we can live with that.

Fixes #697
Ref #617
Actually the comments are fine, we ARE going to remove `QUnit.reset`. We can still keep the fixture reset in a private function.