boards: nrf_bsim: bst hooks: Call tests delete only once

bst_delete() is called from an ON_EXIT hook, which can be called
several times if an exit hook itself fails and calls exit on
its own.
This is not what most tests destructors will expect, and they
may do weird things in this case. So let's not call into it again.

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