Better test for #1181
Better test for #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[_,_]`.

som-snytt/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,lampepfl/dotty,lampepfl/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,dotty-staging/dotty,som-snytt/dotty,som-snytt/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty