Bluetooth: Tester: Added flag parameter to CAP stop cmd

Added a flag parameter so that it is possible
to use the bt_cap_initiator_unicast_audio_stop to perform
disable+stop without releasing the streams by setting the
RELEASE flag.

This allows us to use the bt_cap_initiator_unicast_audio_stop
function to just disable streams without releasing them,
as that is requested by some PTS tests such as
CAP/INI/UST/BV-40-C.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>