tests: root-only test would fail to remove its loop device

tests: root-only test would fail to remove its loop device

* tests/t8001-loop-blkpg.sh: Ironically, when skipping this test,
there was a race condition where in more than 7 out of 10 trials,
this test's "losetup -d $dev" command would fail with "device in use".
Hence, with enough testing, I would eventually use up all loop devices
and start getting new test failures due to losetup *creation* failure.