D-Bus testing: check sync result on both sides of TestLocalSync.testSync

Checking that the item still exists on the side where it was created
is okay, but it is more important to check that it also reached the
other side.

test/test-dbus.py

@@ -4264,6 +4264,8 @@ END:VCARD''')
         loop.run()
         self.assertEqual(DBusUtil.quit_events, ["session " + self.sessionpath + " done"])
         self.checkSync()
+        input = open(xdg_root + "/client/0", "r")
+        self.assertIn("FN:John Doe", input.read())
         input = open(xdg_root + "/server/0", "r")
         self.assertIn("FN:John Doe", input.read())