cross-thread QTimer::singleShot() potential memleak

The tst_QTimer::crossThreadSingleShotToFunctor() test leaks massive amounts of objects (almost all of the 10'000 timers created in the loop). Volker's recent change (87535e4e4337596066258e361143cb9906e89512) helps, by leaking less, but it doesn't fix the issue.

Log attached, which, at the end, shows that this is the only function with a problem, all other test functions are clean.