Bump phpunit/phpunit from 7.4.0 to 7.5.1

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 7.4.0 to 7.5.1.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/master/ChangeLog-7.5.md)
- [Commits](sebastianbergmann/phpunit@7.4.0...7.5.1)

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