Drop old syntax in more files

Author: odersky

library/src/scala/compiletime/testing/ErrorKind.scala

@@ -2,6 +2,6 @@ package scala.compiletime.testing
 
 /** An error can be either a parse-time or a typecheck-time */
 sealed trait ErrorKind  // TODO make this enum, so far not doable because ScalaJS compilation fails on it
-object ErrorKind
+object ErrorKind:
   case object Parser extends ErrorKind
   case object Typer extends ErrorKind