check: use full paths for diff on error

check: use full paths for diff on error

i don't run fstests from the source directory, so when I get a
golden image mismatch the relative path to the golden output is
not useful:

(Run 'diff -u tests/generic/013.out /home/dave/src/xfstests-dev/results//xfs_64k/generic/013.out.bad'  to see the entire diff)

Change the output to emit the real path for the golden out so this
can be cut and pasted and run from anywhere.

(Run 'diff -u /home/dave/src/xfstests-dev/tests/generic/013.out /home/dave/src/xfstests-dev/results//xfs_64k/generic/013.out.bad'  to see the entire diff)

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>