Use more mocking instead of allowing to override final classes

[carlobeltrame]

If a package vendor specifies a class as final, they really do not expect the class to ever be overridden. Also, adding this package to require-dev allows us to do so in a dev environment, even accidentally outside of tests, and that code will only fail when running in production.
Instead of trying to enable mocking SearchFilter, we can just instantiate it and mock a little more in another place. The test testNormalizeDoesntReplaceWhenStrategyIsNotExact was flawed, because the search strategy is always forced to be "exact" when filtering by an association. For this reason, I removed that test, because it does not make sense to test a primitive field filter in the context of RelatedCollectionLinkNormalizer. This normalizer is only concerned with associations, not with primitive fields.

[usu]

I can see the potential danger, so if we find good alternatives, good for me.

Also, adding this package to require-dev allows us to do so in a dev environment, even accidentally outside of tests

The package is only activated in the test bootstrap, not in dev environment. But yeah, theoretically, if you extend a final class and only look a the test results, they could run fine even though the dev & prod environment would not run. My expectation would be though, that both psalm & phpstan would scream about this. Haven't tested though.

Currently, the following test is still broken:
89) App\Tests\Serializer\Normalizer\RelatedCollectionLinkNormalizerTest::testNormalizeDoesntReplaceWhenFilterIsNotSearchFilter
PHPUnit\Framework\MockObject\ClassIsFinalException: Class "ApiPlatform\Doctrine\Orm\Filter\DateFilter" is declared "final" and cannot be doubled

[carlobeltrame]

Currently, the following test is still broken: 89) App\Tests\Serializer\Normalizer\RelatedCollectionLinkNormalizerTest::testNormalizeDoesntReplaceWhenFilterIsNotSearchFilter PHPUnit\Framework\MockObject\ClassIsFinalException: Class "ApiPlatform\Doctrine\Orm\Filter\DateFilter" is declared "final" and cannot be doubled

Thanks, fixed