Assert all test files are cleaned up in CI

[iluuu1994]

Closes GH-8427

[mvorisek]

it should/can target PHP-8.0 as such test expectation is desired there too

[iluuu1994]

Since nightly also runs this validation for each branch, we'll either need to:

1. Clean up files in those branches too
2. Just validate this on master

I'll likely opt for 2.

[TimWolla]

I'll likely opt for 2.

I think this is reasonable. The older branches will naturally rotate out and once they are gone, the check can be enabled for all branches.

[mvorisek]

Clean up files in those branches too

I am the author of #8427 and the issue was an issue with ext/mysqli/tests/clean_table.inc in PHP 8.1. And the fix was already merged. I discussed this in the PR desc...

So you can change the target to PHP 8.1, trigger CI, if green, then do the same for PHP-8.0 and if green, merge :)