tests: Bluetooth: Audio: Move role specific mocks files

For the LE Audio unittests there exists a few mock files
that implement mock versions, or callbacks, for some of the
roles and features tested.

These have been moved to where they are actually used,
reducing the scope of these files.

This both allows the individual tests to implement their own
versions of it, but more importantly it prevents issues when
adding tests for these roles. For example, due to the
bap_unicast_client.c mock file, it is impossible to implement
unit tests for the unicast client, as the functions are already
defined.

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