pytests: test_modem_manager: handle any mmcli modem number

the pytest hardcodes out = subprocess.check_output(["mmcli", "-m", "0"]) so only checks for modem number 0. It can happen for the modem to be number 1 (or higher) if it disconnects and reconnects. This can happen not only on HKS toggle.

So whatever sudo mmcli -L| cut -d / -f 6|cut -d ' ' -f 1 is in python could be added instead of hard coding 0.