xfstests: generic/193 runs tests in wrong location

xfstests: generic/193 runs tests in wrong location

generic/193 runs the test in $here - the root of the xfstests source
tree/installation. IOWs, it doesn't test the filesystem on either
the TEST_DIR or SCRATCH_MNT, and so it not testing the filesystem
we think it is testing. Bad. Fixing this is the majority of the
change - introducing $test_root and $test_user for the files with
different owners, and then redirecting error output and filtering
the output appropriately.

And then add checks that truncate clears the suid/sgid bits
appropriately, something that has never been tested on XFS (and
likely other filesystems) so will cause kernels between 3.1 and 3.9
to assert fail as Dave Jones has recently reported.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>