tests: Bluetooth: Split CAP initiator tests to unicast and broadcast

Split the cap_initiator_test.c to two new files
cap_initiator_broadcast_test.c and
cap_initiator_unicast_test.c as the two barely had any
common functionality, and the file was getting too large.

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