tests: try harder to clean up scsi_debug

tests: try harder to clean up scsi_debug

Tests were failing because the previous test gave up while trying to
unload the scsi_debug module.  Try longer to remove it, and then throw
in a call to udevadm settle for good measure.
* tests/t-local.sh: Try harder to release resources here, rather than...
* tests/t9040-many-partitions.sh: ...in this one test.
* tests/t-local.sh (require_scsi_debug_module_): Now that
cleanup requires "udevadm settle", add that requirement here:
call require_udevadm_settle_.