cmux: Implement unit tests

Bottom line is that we have code coverage for the cmux/ewma code but no unit tests actually testing validity/correctness.

This ticket it to implement tests as much as we can.

#29698 (moved) is the first addition in a long time to the cmux/ewma subsystem and we have no infrastructure whatsoever to test it. We can't merge it until we have proper testing. Thus, putting this ticket as a child.