tests: modem: cmux: add missing timeout param

Add the missing timeout parameter to `modem_pipe_open()` and
`modem_pipe_close()` calls.
10 seconds is the default value used in the Zephyr tree.

Fixes a regression introduced in
https://github.com/zephyrproject-rtos/zephyr/pull/74325

.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>