Bluetooth: audio: test: Refactor unit test

The unittest for cap commander is refactored, so that tests
for volumecontrol is in its own module.
This avoids the problem of having all tests in one huge file.

Due to their small size the tests for registering callbacks and
discovery haven't been put into their own module.

Signed-off-by: Andries Kruithof <andries.kruithof@nordicsemi.no>