sdlmixer_test: Accept a lower value for sample MP3 duration

[smcv]

All the decoders I've tried think sdl2/test/resources/soundtest.mp3 is approximately 0.15 seconds long.

Resolves: https://github.com/py-sdl/py-sdl2/issues/243

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PR Description

This fixes a test failure with SDL_mixer 2.6.1 compiled to use libmpg123 for MP3. Debian's versions of extract(1) and Audacity agree that the test file is about 0.15 seconds long.

Merge Checklist

• [x] the PR has been reviewed and all comments are resolved
• [x] all CI checks pass
• [x] (if applicable): the PR description includes the phrase closes #<issue-number> to automatically close an issue
• [ ] (if applicable): bug fixes, new features, or API changes are documented in news.rst

[a-hurst]

Thanks so much! Can you add a quick NOTE comment above the changed line to the effect of "file actually ~0.15s, incorrectly reported as > 0.2s by dr_mp3 backend"? Just so I know to revisit the upper bound if it gets fixed upstream.

[smcv]

Can you add a quick NOTE comment above the changed line

Done

[a-hurst]

@smcv Fantastic, thanks again!