tests bsim bap_unicast_audio: Avoid randomness failure

The current seed causes a failure with the updated HW models
due to a change in the random draw, that happens to unluckily
cause the test to fail.
Avoide it with a different random seed.
This is due to a known issue in the controller, which is
being worked on by the maintainer.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>