Add pos testcase

Author: Linyxus

tests/pos/gadt-hkt-bounds.scala

@@ -0,0 +1,7 @@
+type Const = [X] =>> Int
+
+trait Expr[F[_]]
+case class ConstExpr() extends Expr[Const]
+
+def foo[F[_], A](e: Expr[F]): F[A] = e match
+  case _: ConstExpr => 0