overlay/018: print hardlink content to golden output

diff may skip comparing content of files with identical st_ino/st_dev.
Overlayfs stat(2) may return same st_dev/st_ino for hardlink copy ups,
but it does not mean that read(2) will return the same content.

Convert the test to output hardlink files content to golden output
instead of using diff.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>

tests/overlay/018

@@ -57,7 +57,7 @@ _scratch_mkfs >>$seqres.full 2>&1
 # Create 2 hardlinked files in lower
 lowerdir=$OVL_BASE_SCRATCH_MNT/$OVL_LOWER
 mkdir -p $lowerdir
-echo "patient zero" >> $lowerdir/foo
+echo "zero" >> $lowerdir/foo
 ln $lowerdir/foo $lowerdir/bar
 
 
@@ -91,16 +91,16 @@ bar=$SCRATCH_MNT/bar
 
 FILES="$foo $bar"
 
+echo "== Before copy up =="
+cat $FILES
 record_ino_nlink $tmp.before
 
 # Modify content of one of the hardlinks
-echo "mutated" >> $foo
+echo "one" >> $foo
 
+echo "== After write one =="
+cat $FILES
 check_ino_nlink $tmp.before $tmp.after
 
-# Compare content of files - expect silence
-diff $foo $bar
-
-echo "Silence is golden"
 status=0
 exit

tests/overlay/018.out

@@ -1,2 +1,9 @@
 QA output created by 018
-Silence is golden
+== Before copy up ==
+zero
+zero
+== After write one ==
+zero
+one
+zero
+one