Fix #4820: add test

The code no longer type checks after #5027.

Author: liufengyun

tests/neg/i4820.scala

@@ -0,0 +1,2 @@
+class Foo[A]
+class Bar[A] extends Foo  // error

tests/neg/i4820b.scala

@@ -0,0 +1,5 @@
+trait SetOps[A, +C <: SetOps[A, C]]  {
+  def concat(that: Iterable[A]): C = ???
+}
+
+class Set1[A] extends SetOps // error: should be SetOps[A, Set1[A]]

tests/neg/i4820c.scala

@@ -0,0 +1,2 @@
+trait Foo[A]
+class Bar[A] extends Foo // error