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.
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