tests: bsim: Bluetooth: Add BAP unicast disconnect test

The purpose of this test is to verify that the stack has
the correct behavior if the ACL disconnects while one or
more streams are in the streaming state.

The main point it validates is that unicast server device
can restart advertising using a simple k_work in the
disconnected callback.

The number of connections supported by the test suite has
also been reduced to 3, to avoid some seen issues with
scheduling when CONFIG_BT_MAX_CONN=5.

A part of implementing the test for this purpose, the
unicast client and unicast server have been extended
to also advertise and scan for proper ASCS advertising
data.

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