Async: Fail assertions after existing assert.async flows are resolved

Fail assertions after existing `assert.async` flows are resolved.

Fixes #668
Closes #669
As mentioned in https://github.com/jquery/qunit/issues/668#issuecomment-56538068, this particular test case makes me sad but it complies with the rules we've laid out and is easy to workaround if you want to do something like this.