Add pos testcase for HKT GADT orderings

Author: Linyxus

tests/pos/gadt-hkt-ordering.scala

@@ -0,0 +1,10 @@
+object test {
+  final class HKTVar[+T[_]]
+
+  def foo[F[_], G[_], X](m : HKTVar[G]) = m match {
+    case _ : HKTVar[F] =>
+      val fx : F[X] = ???
+      val gx : G[X] = fx
+  }
+
+}