Fix mock object type hints in comments

We have (doc comment) type hints for \PHPUnit_Framework_MockObject_MockObject in several places in our test base (when we want to have both that and the mocked class as the type hint, so IDEs will autocomplete correctly for both). However, that class name is for an old version of PHPUnit, the new class name is \PHPUnit\Framework\MockObject\MockObject. To preserve the intended effect, we should update all references.