tests: modem: cmux: extend test suite to cover release case

When the CMUX is released from its pipe, it must stop transmitting
data to it, and prevent starting any work which requires
transmitting data like connecting CMUX or opening a DLCI pipe.

This commit extends the CMUX test suite to validate this behavior.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>