Ensure that expired threads have actually finished before attempting
to restart them. Calling start() on a thread that is not yet finished
does nothing.
Add a regression test into tst_qthreadpool that attempts to trigger
reuse of expired threads and verifies that all submitted tasks
execute.
Fixes: QTBUG-72872
Pick-to: 6.2
Change-Id: I2109b628b8a4e91491115dc56aebf3eb249646b5
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
1afd562b0b