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
Closes #698