Composer: bump wp-phpunit/wp-phpunit in the phpunit group

Bumps the phpunit group with 1 update: [wp-phpunit/wp-phpunit](https://github.com/wp-phpunit/wp-phpunit).


Updates `wp-phpunit/wp-phpunit` from 6.4.2 to 6.5.4
- [Commits](wp-phpunit/wp-phpunit@6.4.2...6.5.4)

---
updated-dependencies:
- dependency-name: wp-phpunit/wp-phpunit
  dependency-type: direct:development
  update-type: version-update:semver-minor
  dependency-group: phpunit
...

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