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>

check

@@ -799,7 +799,7 @@ for section in $HOST_OPTIONS_SECTIONS; do
 			else
 				head -n "$DIFF_LENGTH"
 				echo "..."
-				echo "(Run '$diff $seq.out $seqres.out.bad'" \
+				echo "(Run '$diff $here/$seq.out $seqres.out.bad'" \
 					" to see the entire diff)"
 			fi; } | sed -e 's/^\(.\)/    \1/'
 			err=true