Demonstrate what future async support in QUnit should do

This is not intended to be merged, at least not as-is. The idea here is to show what the problem with async assertions is, and what it should look and behave like in the future. 

Running the testsuite from this PR currently results in this output:

![bad](http://bassistance.de/i/b2faeb.png)

This highlights an important issue with async tests right now: Assertions can leak into any other test. In this case I've constructed deterministic tests. In practice, the result is not deterministic and much worse, because the "async assertion" can end up in any other test.

The desired output here would deal with that. So that test #1 would fail with "Expected 1 assertions, but 0 were run", but also have a failing assertion with stacktrace and the original message "this belongs to test #1", in addition to something like "assertion ran outside of its own test context". The same would apply to test #2, except that the assertion here would fail for two reasons. Both "catchAll" tests would be green, having 0 assertions. This is what #374 is about.

This PR also demonstrates a possible replacement for the current `stop()`/`start()` pair, adding `async` and `done` methods to the assertion argument. As demonstrated here, that's a pretty trivial change, though it might get more interesting when combined with the above or the suggestions regarding promises in #534.

@leobalter would be great if you could take a look at this. We can then discuss some details and look into actually implementing it. Maybe @Krinkle has input, too?