tests: Bluetooth: Audio: Add BSIM test of the CAP samples

Adds babblesim tests of the CAP Acceptor and the CAP Initiator
samples for unicast.

This simply checks if at least one stream is connected
and is sending (empty) data.

This modifies the samples to send data as well as counting the
receive ISO data packets.
Ideally the TX would be superflous to verify that ISO is working,
but a missing feature in the Zephyr LL makes it a requirement.

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