Bump phpunit/phpunit from 8.5.8 to 8.5.15

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 8.5.8 to 8.5.15.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-8.5.md)
- [Commits](sebastianbergmann/phpunit@8.5.8...8.5.15)

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