fix(modem): Add CMUX example to target tests #577

Job	Run time
Build (latest, pppos_client)	8m 1s
Host Tests	2m 14s
Build (latest, modem_console)	4m 38s
Run gcovr on esp modem host test	2m 28s
Build (latest, modem_tcp_client)	15m 18s
Build (latest, ap_to_pppos)	10m 31s
Build (latest, simple_cmux_client)	3m 32s
Build (release-v4.2, pppos_client)	55s
Build (release-v4.2, modem_console)	3m 59s
Build (release-v4.2, ap_to_pppos)	4m 1s
Build (release-v4.3, pppos_client)	2m 18s
Build (release-v4.3, modem_console)	5m 33s
Build (release-v4.3, ap_to_pppos)	5m 17s
Build (release-v4.3, simple_cmux_client)	2m 31s
Build (release-v4.4, pppos_client)	5m 44s
Build (release-v4.4, modem_console)	4m 37s
Build (release-v4.4, ap_to_pppos)	7m 20s
Build (release-v4.4, simple_cmux_client)	3m 8s
Build (release-v5.0, pppos_client)	5m 39s
Build (release-v5.0, modem_console)	4m 7s
Build (release-v5.0, modem_tcp_client)	8m 16s
Build (release-v5.0, ap_to_pppos)	8m 7s
Build (release-v5.0, simple_cmux_client)	2m 15s
	2h 0m 29s

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