Add remaining SI-2712 testcases from Scala 2

It turns out that neg/t2712-2.scala actually works in Dotty due to
better type inference.

Author: smarter

tests/neg/t2712-8.scala

@@ -0,0 +1,10 @@
+object Test extends App {
+  class L[A]
+  class Quux0[B, CC[_]]
+  class Quux[C] extends Quux0[C, L]
+
+  def foo[D[_]](x: D[D[Boolean]]) = ???
+  def bar: Quux[Int] = ???
+
+  foo(bar) // error: Found: Test.Quux[Int]  Required: D[D[Boolean]]
+}

tests/pos/t2712-2b.scala

@@ -0,0 +1,18 @@
+package test
+
+class X1
+class X2
+class X3
+
+trait One[A]
+trait Two[A, B]
+
+class Foo extends Two[X1, X2] with One[X3]
+object Test {
+  def test1[M[_], A](x: M[A]): M[A] = x
+
+  val foo = new Foo
+
+  test1(foo): One[X3] // fails in Scala 2 with partial unification enabled, works in Dotty
+  test1(foo): Two[X1, X2]
+}