tests: Bluetooth: CAP: Add verification of CAP unicast start callbacks

Add verification of the CAP Unicast Audio Start callback parameters.
There was a missing part in the BAP Unicast Client mock that did not
set the ISO state correctly, which caused the procedures to fail, but
without the verification that was not caught.

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