modem: modem_cmux: Increase modem cmux buf size

This commit increases the buffer used for commands
in the control channel within an instance of the
modem_cmux module. The buffer was not large enough to
store an MSC command if the optional break signals
where included. This commit fixes the issue and
updates the test suite to use the max size MSC message.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>