Fix #6664: add test

Author: liufengyun

tests/run/i6664/Bomb_1.scala

@@ -0,0 +1,4 @@
+enum Bomb
+{
+    case Kaboom
+}

tests/run/i6664/Detonator_2.scala

@@ -0,0 +1,5 @@
+object Test
+{
+    def boom: Unit = Bomb.valueOf("Kaboom")
+    def main(args: Array[String]): Unit = println("ok!")
+}