tests|lei: fixes for TEST_RUN_MODE=0 and lei oneshot

tests|lei: fixes for TEST_RUN_MODE=0 and lei oneshot

DESTROY callbacks can clobber $?, so we must take care to
preserve it when exiting.  We'll also try to make an effort to
ensure better DESTROY ordering and delete as much as possible
before x_it finishes.

We also need to load PublicInbox::Config when setting up
public inboxes.