Add pos test

Author: Linyxus

tests/pos/gadt-param-unification.scala

@@ -0,0 +1,6 @@
+trait Expr[T]
+final class Lit[T] extends Expr[T]
+
+def foo[X, T1 >: X, T2](m: Expr[T2]): T2 = m match {
+  case _: Lit[T1] => ??? : X
+}