[ffmpeg] race condition in looped playback (discovered through tst_QMediaPlayerBackend being flaky)

    3556: ********* Start testing of tst_QMediaPlayerBackend *********
    FAIL!  : tst_QMediaPlayerBackend::play_playbackLastsForTheExpectedTime(file qrc:/testdata/busMpeg4.mp4, loops 2, rate 1, pause before 1) 'abs(duration - expectedDuration) < 600ms' returned FALSE. (expected duration: 2000ms, actual duration: 1014ms)
       Loc: [/home/qt/work/qt/qtmultimedia/tests/auto/integration/qmediaplayerbackend/tst_qmediaplayerbackend.cpp(1720)]
    Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/tmp/tst_qmediaplayerbackend.VTvzBS.mp4':
      Metadata:
    FAIL!  : tst_QMediaPlayerBackend::play_playbackLastsForTheExpectedTime(file qrc:/testdata/busMpeg4.mp4, loops 2, rate 0.5, pause before 1) 'abs(duration - expectedDuration) < 600ms' returned FALSE. (expected duration: 4000ms, actual duration: 2032ms)
       Loc: [/home/qt/work/qt/qtmultimedia/tests/auto/integration/qmediaplayerbackend/tst_qmediaplayerbackend.cpp(1720)]
    Input #0, wav, from '/tmp/tst_qmediaplayerbackend.uebPqK.wav':
      Duration: 00:00:01.00, bitrate: 705 kb/s
    Totals: 198 passed, 2 failed, 9 skipped, 0 blacklisted, 331294ms
    
    
    13313: ********* Start testing of tst_QMediaPlayerBackend *********
    FAIL!  : tst_QMediaPlayerBackend::play_playbackLastsForTheExpectedTime(file qrc:/testdata/busMpeg4.mp4, loops 2, rate 0.5, pause before 1) 'abs(duration - expectedDuration) < 600ms' returned FALSE. (expected duration: 4000ms, actual duration: 2037ms)
       Loc: [/home/qt/work/qt/qtmultimedia/tests/auto/integration/qmediaplayerbackend/tst_qmediaplayerbackend.cpp(1720)]
    PASS   : tst_QMediaPlayerBackend::cleanupTestCase()
    Totals: 2 passed, 1 failed, 0 skipped, 0 blacklisted, 2547ms
    
    
    14194: ********* Start testing of tst_QMediaPlayerBackend *********
    FAIL!  : tst_QMediaPlayerBackend::play_playbackLastsForTheExpectedTime(file qrc:/testdata/busMpeg4.mp4, loops 2, rate 0.5, pause before 1) 'abs(duration - expectedDuration) < 600ms' returned FALSE. (expected duration: 4000ms, actual duration: 2036ms)
       Loc: [/home/qt/work/qt/qtmultimedia/tests/auto/integration/qmediaplayerbackend/tst_qmediaplayerbackend.cpp(1720)]
    PASS   : tst_QMediaPlayerBackend::cleanupTestCase()
    Totals: 2 passed, 1 failed, 0 skipped, 0 blacklisted, 2545ms