Add testcase for stripping refinements in GADT

Author: Linyxus

tests/pos/gadt-strip-refinement.scala

@@ -0,0 +1,6 @@
+trait Tag[T] { type X }
+class IntTag extends Tag[Int]
+
+def foo[T](x: Tag[T]): T = x match {
+  case _: IntTag { type X = String } => 0
+}