[messages] Fix analyzer unit test failure output.

When an analyzer unit test fails due to unexpected diagnostic errors,
the failure message contains text like this:

    To accept the current state, expect:
      error(<diagnostic code>, <offset>, <length>),
      ...

This change fixes the "diagnostic code" part of the output so that it
uses the new `diag` prefix, rather than trying to refer to static
constants in `DiagnosticCode`-derived classes, e.g.:

    To accept the current state, expect:
      error(diag.missingConstFinalVarOrType, 50, 1),
      error(diag.expectedToken, 50, 1),

This paves the way for removing the `DiagnosticCode`-derived classes
entirely.

Change-Id: I6a6a6964176a29fa7c50bc06be5b79898da538b8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/462520
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer/test/generated/test_support.dart

@@ -499,10 +499,9 @@ extension on DiagnosticCode {
   /// to change test expectations to match the current behavior.
   ///
   /// For example, if the unique name is `TestClass.MY_ERROR`, this method will
-  /// return `TestClass.myError`.
+  /// return `diag.myError`.
   String get constantName => switch (uniqueName.split('.')) {
-    [var className, var snakeCaseName] =>
-      '$className.${snakeCaseName.toCamelCase()}',
+    [_, var snakeCaseName] => 'diag.${snakeCaseName.toCamelCase()}',
     _ => throw StateError('Malformed DiagnosticCode: $uniqueName'),
   };
 }