Add Exhaustivity Test for Enums with mixins

[Jamie Thompson]

sealed trait Pretty { self: Color => }      
sealed trait Dull { self: Color => }
enum Color  {
  case Pink extends Color with Pretty
  case Violet extends Colour with Pretty
  case Red extends Color with Dull
}

def describe(c: Colour) = c match {
  case Colour.Pink | Colour.Violet => "Pretty!"
  case Colour.Red                  => "Dull!"
}

def describePartial(c: Colour) = c match { // error: would fail on case Red
  case Colour.Pink | Colour.Violet => "Pretty!"
}

def describePretty(c: Pretty) = c match {
  case Colour.Pink | Colour.Violet => "Pretty!"
  case Colour.Red                  => "Pretty!" // error: Red not instance of Pretty
}

def describePrettyPartial(c: Pretty) = c match { // error: would fail on case Violet
  case Colour.Pink => "Pretty!"
}

def describeDull(c: Dull) = c match {
  case Colour.Pink => "Dull!" // error: Pink not instance of Dull
  case Colour.Red  => "Dull!"
}

from scala/scala3#8203