Tests sometimes failed because a PortRPC method handler was not called
after the fixed message delivery delay that the tests allowed.

I have not been able to reproduce locally, but attempt to resolve the
issue by waiting for a specific message to be delivered to the
MessagePort instead of waiting a fixed amount of time.

For some test cases which assert that a message was *not* delivered, a
fixed delay is still used. This is acceptable because the test will
still pass in the rare cases where the fixed delay is too short.