Bluetooth: CAP: Add support for doing just disable for unicast stop

The unicast_stop function is changed to primarily do a
BAP disable instead of a release, with optional
support for releasing the streams once they have been disabled.

This also adds unittests for the procedure which also
allow us to remove the invalid param testing from the BSIM tests.

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