Fix ShowPretty tests

Define companion object as scala/scala3#15391
isn't released yet

Author: TimWSpence

core/src/test/scala-3/cats/derived/adtdefns.scala

@@ -217,6 +217,8 @@ object TestDefns:
   final case class IntLeaf(t: Int) extends IntTree
   final case class IntNode(l: IntTree, r: IntTree) extends IntTree
 
+  object IntTree {}
+
   sealed trait GenericAdt[A]
   final case class GenericAdtCase[A](value: Option[A]) extends GenericAdt[A]