tests/bsim broadcast_audio: Increase realtime timeout

This test has been seen failing in older slower computers
due to timeouts, let's increase the timeout so we don't
break in those cases.
Note this timeout is just a safety to eventually kill
hung simulations even if nobody presses Ctrl+C.

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