Fix #6677: add test

Author: liufengyun

tests/run/i6677/Enum_1.scala

@@ -0,0 +1,3 @@
+enum Foo[A] {
+  case Bar extends Foo[Int]
+}

tests/run/i6677/Test_2.scala

@@ -0,0 +1,5 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    println(Foo.Bar)
+  }
+}