formula: update test_fixtures path
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- [x] Have you followed the guidelines in our [Contributing](https://github.com/Homebrew/brew/blob/master/CONTRIBUTING.md) document?
- [x] Have you checked to ensure there aren't other open [Pull Requests](https://github.com/Homebrew/brew/pulls) for the same change?
- [x] Have you added an explanation of what your changes do and why you'd like us to include them?
- [ ] Have you written new tests for your changes? [Here's an example](https://github.com/Homebrew/homebrew/pull/49031).
- [ ] Have you successfully run `brew tests` with your changes locally?
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Fix regression caused by #1364.
Example of the regression: https://bot.brew.sh/job/Homebrew%20Core/11621/version=any_sierra/testReport/junit/brew-test-bot/sierra/test_mpv/.