Don't use /tmp for hosting loopback images

Don't use /tmp for hosting loopback images

/tmp might be small, might not support files larger than 2GB,
etc, so trying to host loopback images of 100GB filesystems
will break in some situations. We should use $TEST_DIR for the
images.

Note: remounting of loopback images has a major bug (in mount)
that "leaks" loopback device references. Hence all the hackery
with losetup to work around this.
Merge of master-melb:xfs-cmds:31169a by kenmcd.

  Don't use /tmp for hosting loopback images