Test case: tst_QMediaPlayerBackend::setActiveSubtitleTrack_switchesSubtitles has failed 13 times, crashed 0 and was flaky 13 times last week.

The test has blocked around 13 Gerrit changes past week

Coin log excerpt from failed run:
agent:2025/09/15 07:53:47 build.go:413: FAIL! : tst_QMediaPlayerBackend::setActiveSubtitleTrack_switchesSubtitles(while playing, subtitles start at zero) Compared lists have different sizes.
agent:2025/09/15 07:53:47 build.go:413: Actual (subtitleSink.subtitles) size: 6
agent:2025/09/15 07:53:47 build.go:413: Expected (expectedSubtitles) size: 4
agent:2025/09/15 07:53:47 build.go:413: Loc: [/Users/qt/work/qt/qtmultimedia/tests/auto/integration/qmediaplayerbackend/tst_qmediaplayerbackend.cpp(4453)]
agent:2025/09/15 07:53:47 build.go:413: Input #0, matroska,webm, from '/var/folders/0r/vd385j117f949qnfm4h_539w0000gn/T/tst_qmediaplayerbackend.dlEVbo.mkv':
agent:2025/09/15 07:53:47 build.go:413: Metadata:
agent:2025/09/15 07:53:47 build.go:413: ENCODER : Lavf60.16.100
agent:2025/09/15 07:53:47 build.go:413: Duration: 00:00:15.05, start: 0.000000, bitrate: 81 kb/s
agent:2025/09/15 07:53:47 build.go:413: Stream #0:0: Video: h264 (High 4:4:4 Predictive), yuv420p(tv, bt709, progressive), 1920x1080 [SAR 1:1 DAR 16:9], 25 fps, 25 tbr, 1k tbn (default)
agent:2025/09/15 07:53:47 build.go:413: Metadata:
agent:2025/09/15 07:53:47 build.go:413: title : One
...

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