Fix time-stamp tests in tst_qmediaplayerbackend

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>

tests/auto/integration/qmediaplayerbackend/tst_qmediaplayerbackend.cpp

@@ -749,6 +749,10 @@ void tst_QMediaPlayerBackend::seekPauseSeek()
     QTRY_COMPARE(player.state(), QMediaPlayer::PausedState); // it might take some time for the operation to be completed
     QTRY_VERIFY(!surface->m_frameList.isEmpty()); // we must see a frame at position 7000 here
 
+    // Make sure that the frame has a timestamp before testing - not all backends provides this
+    if (surface->m_frameList.back().startTime() < 0)
+        QSKIP("No timestamp");
+
     {
         QVideoFrame frame = surface->m_frameList.back();
         const qint64 elapsed = (frame.startTime() / 1000) - position; // frame.startTime() is microsecond, position is milliseconds.