fix(modem): Add CMUX example to target tests #581

Job	Run time
Host Tests	2m 8s
Build (latest, pppos_client)	2m 31s
Build (latest, modem_console)	2m 30s
Run gcovr on esp modem host test	2m 5s
Build (latest, modem_tcp_client)	2m 32s
Build (latest, ap_to_pppos)	2m 29s
Build (latest, simple_cmux_client)	2m 28s
Build (release-v4.2, pppos_client)	1m 0s
Build (release-v4.2, modem_console)	2m 29s
Build (release-v4.2, ap_to_pppos)	2m 27s
Build (release-v4.3, pppos_client)	2m 20s
Build (release-v4.3, modem_console)	2m 28s
Build (release-v4.3, ap_to_pppos)	2m 29s
Build (release-v4.3, simple_cmux_client)	2m 22s
Build (release-v4.4, pppos_client)	2m 28s
Build (release-v4.4, modem_console)	2m 28s
Build (release-v4.4, ap_to_pppos)	2m 28s
Build (release-v4.4, simple_cmux_client)	2m 29s
Build (release-v5.0, pppos_client)	2m 28s
Build (release-v5.0, modem_console)	2m 28s
Build (release-v5.0, modem_tcp_client)	2m 28s
Build (release-v5.0, ap_to_pppos)	2m 29s
Build (release-v5.0, simple_cmux_client)	2m 23s
	54m 27s

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