tst_QThreadPool::tryStartCount(): Wait until tasks have finished.

Replace hardcoded interval by QTRY_COMPARE() ensuring that all tasks have
finished, since otherwise QThreadPool::tryStart() fails in the next run.

Remove blacklisting for MSVC2012 since it is not compiler-specific.

Task-number: QTBUG-49654
Change-Id: If49b6eb9ef4da4ba2911839166c397d84c175e00
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@theqtcompany.com>