CI: Only enable Whoops when it is tested

[lukasbestle]

This first step already helps us to make CI succeed on PHP 8.0. I'll still work on the other quick fix as we discussed, but that may take a little longer.

This PR …

Refactoring

• Whoops is now generally disabled during PHPUnit test runs to reduce memory usage during tests

Breaking changes

None

Ready?

• Unit tests for fixed bug/feature
• In-code documentation (wherever needed)
• Tests and checks all pass

For review team

• Add changes to release notes draft in Notion
• Add to website docs release checklist (if needed)

[distantnative]

Works for me locally and I implementation looks quite clean.

Didn't quite understand the Assert/Deprecated PHPUnit stuff, but guess that's intended/fine.

[lukasbestle]

The Error\Deprecated and Error\Warning classes are exceptions thrown by PHPUnit when a PHP (deprecation) warning was triggered. So by catching those, we can test for the warnings.

I first used $this->expectException(Deprecated::class), however that lead to a PHPUnit deprecation warning by itself because PHPUnit 10 will no longer convert warnings to exceptions. That's why the try/catch is necessary for now. We'll need yet another solution once we switch to PHPUnit 10: sebastianbergmann/phpunit#5062

Assert::fail() throws an exception that makes the test fail. That's needed to mark the test as failed if the catch didn't run as expected.