... by supplying a TestThreadPool that waitsForDone() with a defined
timeout in the dtor.
When tests hang, this will now print an intelligible message instead
of just the generic watchdog-killed-process one.
Also replace all QVERIFY(waitForDone()) with the same code used in
TestThreadPool's dtor and add a comment in a place we'd rather not use
these tools.
Pick-to: 6.5 6.2
Change-Id: Ifd2b3372eb7c7337a3ba77d003e45dcd77e23545
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>