Add test to expand dn to 'display: none' for css

unittest.vim

@@ -730,6 +730,10 @@ finish
           'query': "{c#dba.7$$$$}",
           'result': "{color: rgb(221, 187, 170, 0.7);}",
         },
+        {
+          'query': "{dn$$$$}",
+          'result': "{display: none;}",
+        },
       ],
     },
   ],