tests: avoid a race condition in the scsi_debug-using tests

tests: avoid a race condition in the scsi_debug-using tests

* tests/t-local.sh (new_sdX_): New function.
(scsi_debug_setup_): Use new_sdX_ to find the just-created device name,
rather relying on the list of file names in /sys/block/sd* changing.
Sometimes the list would not change, which would lead to tests failing
with I/O errors when run in parallel.  Thanks to Brian Lane for the
tip that /sys/block/sd*/device/model contains "scsi_debug" if there
is a device created by our "modprobe scsi_debug" command.