Add a test for global init checker (scala#21694)

Add a test for global init checker

Author: olhotak

tests/init-global/warn/Color.scala

@@ -0,0 +1,28 @@
+enum Color:
+  case None, White, Black
+
+enum Player:
+  case Black, White
+
+  // Explanation: See the desugaring below
+  val color: Color =
+    if this == Player.Black  // warn
+    then Color.Black
+    else Color.White
+
+// From the desugaring of Player, we can see the field `Player.Black` is not yet
+// initialized during evaluation of the first `new Player`:
+//
+//     class Player:
+//         val color: Color =
+//            if this == Player.Black ...
+//
+//     object Player:
+//          val Black: Player = new Player     // <--- problem
+//          val White: Player = new Player
+//
+//
+//  The complex desugaring makes it difficult to see the initialization
+//  semantics and it is prone to make such hard-to-spot mistakes.
+//
+// Note: The desugaring above is simplified for presentation.