document_source_change_stream_test.cpp is already quite large (5694 loc at the moment), and we will need to add many more tests for change stream stages in SPM-1941. Adding more tests will increase the size of the file even more.
The new tests to be added will need the same change stream stage test fixtures that are provided by [document_source_change_stream_test.cpp, e.g. ChangeStreamStageTest and ChangeStreamStageTestNoSetup.
We should move these fixtures out into separate files (.h and .cpp) so they can be accessed from the existing tests, but also from new tests for change stream stages.