Re-enable t-topology-support.sh

Re-enable t-topology-support.sh

Reintroduce split teardown (teardown() calls teardown_devs()) because
t-topology-support.sh only needs the teardown_devs() subset of the full
teardown() between each iteration of the topology tests -- in particular
the $TESTDIR must not get removed between each topology test iteration.

prepare_loop() must return if prepare_scsi_debug_dev() already
established $LOOP.

Also fix (and simplify) the unsafe scsi-debug device discovery in
prepare_scsi_debug_dev().