Prepare the change stream code for tests in the unified format

Currently we cannot just put a change-streams.json file in the unified test format somewhere in the driver repo and expect it to be run because:

1. There are no test runners (neither sync nor reactive) for change streams.
2. There is no location where unified test files can be put. If put in driver-core/src/test/resources/change-streams, they are going to be picked up by AbstractChangeStreamsTest and result in a failure because this runner does not understand the unified syntax.