tests/bsim mesh: Increase realtime timeout

One of these tests 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>