Add test

Author: liufengyun

tests/neg-custom-args/isInstanceOf/html.scala

@@ -10,6 +10,7 @@ object HTML:
       attrs.filter(_ != Nil).foreach{
         case s: Seq[AppliedAttr] =>
           s.foreach(sb.append(" ").append)
+        case s: Seq[Int] => // error
         case e: AppliedAttr =>
           sb.append(" ").append(e)
       }