Backport "simplify trees that appear in tastycheck test" to LTS (#22082)

Backports #21274 to the 3.3.5.

PR submitted by the release tooling.
[skip ci]

tests/pos/i21154/Z.scala

@@ -3,7 +3,9 @@
 // in the original issue https://github.com/scala/scala3/issues/21154, the non-deterministic tasty
 // depends on the order of compilation of files, the use-site (A.scala) has to come first,
 // and the file defining the enum has to come second (Z.scala), A.scala in namer will force Z to complete.
-enum Z:
-  case AOptions()
-  case BOptions()
-  case COptions()
+sealed trait Z
+
+object Z:
+  class AOptions() extends Z
+  class BOptions() extends Z
+  class COptions() extends Z