CTest: make sure never to report negative test times (#14132)

Because of clock scew between processors or just because of someone changing
the system time the end timestamp may be before the start time. Reporting a
negative time doesn't any sense, just report zero there as it already happens
for really fast tests.
Merge topic 'positive-test-times' into next

e319e32 CTest: make sure never to report negative test times (#14132)