Update dependency phpunit/phpunit to v8.5.15

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
phpunit/phpunit (source)	require-dev	patch	8.5.14 -> 8.5.15

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Release Notes

sebastianbergmann/phpunit

### [`v8.5.15`](https://togithub.com/sebastianbergmann/phpunit/compare/8.5.14...8.5.15) [Compare Source](https://togithub.com/sebastianbergmann/phpunit/compare/8.5.14...8.5.15)

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