snapshot: Fix tests/virsh-snapshot

Creating an 'exp' output file, but never comparing it against the
actual output, does not actually constitute testing the output. :)

Fixes: 280a2b41Signed-off-by: NEric Blake <eblake@redhat.com>

tests/virsh-snapshot

@@ -207,6 +207,8 @@ Descendants:    0
 Metadata:       yes
 
 EOF
+sed '1,/^virsh #/d; /virsh #/d' < out > out.cooked || fail=1
+compare exp out.cooked || fail=1
 
 cat <<EOF > exp || fail=1
 error: invalid argument: parent s3 for moment s2 not found