build(deps-dev): bump phpunit/phpunit from 8.5.14 to 8.5.15

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 8.5.14 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@8.5.14...8.5.15)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]