Refactor/modernize test suite setup
As I've been seeing new issues popping up elsewhere with some of the recent changes in PHP 8.1, I had a look at the latest test run for this repo and as I feared, the tests break on PHP 8.1 due to PHPUnit 7.5 being used.
See: https://github.com/squizlabs/PHP_CodeSniffer/runs/3118862021?check_suite_focus=true
As PHPUnit 7.5 is no longer supported, this will not be fixed on the PHPUnit side of things.
I think a refactor of the test suite setup is in order to allow it to be compatible with a wider range of PHPUnit versions, preferably without BC-break in the setup as quite a lot of external standards use the PHPCS native abstract test classes for their tests as well.
While the PHP 8.1 issues have been solved by now, I'm going to leave this issue open, as IMO a refactor of the test suite setup may still be a good idea and make the setup more sustainable in the long run.
The setup as is, with the AllTests, the dynamic test suite creation etc seems to be based on a really old PHPUnit version and while it still works, the framework as-is, is creaking more and more with each new PHP version.