QLibrary intentionally does not unload on destruction, so failing tests
may leave libraries already loaded and cause further tests to fail
because of that. So add a cleanup() method to unload everything we may
have loaded.
Note that QLibrary::unload() sets its state to NotLoaded after one
successful call, so we must recreate the object in case it had been
load()ed multiple times.
Pick-to: 6.2 6.4
Change-Id: I12a088d1ae424825abd3fffd171d133c678f910a
Reviewed-by: Marc Mutz <marc.mutz@qt.io>