Fix compatibility with PHPUnit 11

[derrabus]

This PR fixes a couple of compatibility issues with PHPUnit 11:

• Data provider methods must be static.
• The mock builder does not have a setMethods() method anymore.
• Abstract test classes must not use the suffix Test.

Note: The tests are still run with PHPUnit 8.5 and 9.6 though. The PR merely enables us to use PHPUnit 11 in the future.

[xabbuh]

Thank you Alexander.