[PhpUnitBridge] Added ClassExistsMock #28931
I've thought about this before, and bumped into it again when trying to test #28898
This PR allows to mock
Sadly, the docs issue is not very actionable because the description of this PR is a bit lacking. Please update it to explain which changes would this require for full Symfony apps and for apps that use this as a stand-alone component.
If no changes are needed, please add some before/after examples or show what can we do now that wasn't possible before. Thanks!