Refactor tests to support PHPUnit 10 [2.2]

[terabytesoftw]

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%.
• Add PHPStan for tests.