Not all backends supports or have an implementation that sets the
time-stamp on the video frames, so check if we do have valid time-stamps
before running any tests that depends on it.

Change-Id: I53926022a02cd87d23c493f7b392c4eb4c45a55e
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>