Fix #13491: Add regression test

[OlivierBlanvillain]

No description provided.

[smarter]

LGTM although it might be useful to identify which PR fixed this to backport it to 3.1.0 since it fixes a regression.

[dwijnand]

How come?

[OlivierBlanvillain]

The pickling test failure can be minimized to the following:

object Test {
  type M[X] = X match { case Int => String }
  def a: Any { type Out = M[Int] } = a
}

The failure comes from the refinement type, which is printed as
scala.Any{Out = scala.String} before pickling, and
scala.Any{Out = Test.M[scala.Int]} after pickling...

I'll open a new issue from that minimization.

[OlivierBlanvillain]

Breakage bisected to #12773, fix bisected to #13253.