Refactor tests to support PHPUnit 10 [2.2]

We currently use PHPUnit `9.6` in master (PHP 7.3) and plan to adopt PHPUnit `10` in the `2.2` branch (PHP 8.1+):

- [ ] Remove abstract test classes that are no longer necessary.
- [ ] Mark data providers as static, as required by PHPUnit 10.
- [ ] Separate stubs and helper classes from test classes (e.g. validator tests currently define stub classes inline).
- [ ] Raise code coverage 100%.