Better test for scala#1181

Now that we implement partial higher-order unification (SI-2712 fix)
i1181.scala will compile even if `Alias[Int]` gets dealiased to
`(Int, Int)` because we can unify the latter with `M[_]` where
`M = [X] -> (Int, X)`.

The new test will only succeed if `Alias[Int, Int]` is not dealiased
because we will never unify `Foo[Int]` with `M[_,_]`.

tests/pos/i1181b.scala

@@ -0,0 +1,11 @@
+class Foo[A]
+
+object Test {
+  def foo[M[_,_]](x: M[Int,Int]) = x
+
+  type Alias[X,Y] = Foo[X]
+  val x: Alias[Int,Int] = new Foo[Int]
+
+  foo[Alias](x) // ok
+  foo(x)
+}