tests: Bluetooth: ASCS: Fix various issues in ASCS unit tests

The ASCS unit tests had various errors after adding support for
dynamic registration.
Several tests did not properly clean up after failure, causing other
tests to fail when they shouldn't.

Moved the register tests to their own file as they should not
do the register in the "before" function.

The test_ascs_unregister_with_ases_in_config_state test was also
removed, as it had both issues and the state that it wants to test
cannot be reached with the current API - It is not possible to
put an ASE in the configured state without callbacks,
and registered callbacks prevents us from calling
bt_bap_unicast_server_unregister to trigger the case as that can
only be done if callbacks are unregistered. Since unregistering
callbacks also puts all ASEs to the idle state, it is not possible
to call bt_bap_unicast_server_unregister for a non-idle ASE.

The testcase.yaml was also missing some Kconfig options to
properly enable the client tests.

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