Add error assertions

Author: Sebastian Nadorp

tests/neg/enum-repeated-extend-4470.scala

@@ -1,10 +1,10 @@
 object RepeatedExtendEnum {
 
-  enum Maybe[T] {
+  enum Maybe[T] { // error // error
     case Foo extends Maybe[Int]
   }
 
-  enum Maybe[T] {
+  enum Maybe[T] { // error
     case Foo extends Maybe[Int]
   }
 }