Add mp3 test audio file

[jmagman]

flutter/plugins#4639 added support for the video_player plugin to play mp3s. Add an mp3 test file to be used by the plugin tests, similar to the existing mp4 test files like bee.mp4

mp3 is of a rooster: https://pixabay.com/sound-effects/rooster-crowing-7027/

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• I listed at least one issue that this PR fixes in the description above.
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• I added new tests to check the change I am making, or this PR is test-exempt.
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[jmagman]

Context: want to test fix for regression at flutter/flutter#97356

[Hixie]

[jmagman]

I can also choose something less startling, if we're worried about heart attacks during future integration tests.
https://pixabay.com/sound-effects/search/theme/nature/

https://pixabay.com/sound-effects/short-pigeon-wing-flap-14625/
https://pixabay.com/sound-effects/underwater-6236/
https://pixabay.com/sound-effects/birds-taking-off-fly-6277/

[jmagman]

I can change it later if the rooster is too much. 🐓