[analyzer_testing] Testing warnings is not valid anymore

[FMorschel]

After https://github.com/dart-lang/sdk/issues/61793, I can't test warnings correctly. Because of what I mentioned in https://github.com/dart-lang/sdk/issues/61928. We are always using Registry.ruleRegistry.registerLintRule and this internally casts the DiagnosticCode to LintCode.

  @mustCallSuper
  @override
  void setUp() {
    // TODO(srawlins): In a major release, change this logic to just ensure that
    // `rule` has been set to an analysis rule instance other than
    // `_SentinelRule`.
    if (rule is _SentinelRule) {
      if (analysisRule == 'sentinel') {
        throw StateError('The `rule` field must be set in the `setUp` method.');
      }
      // The developer is using the deprecated `analysisRule` to set the
      // rule-under-test.
      _analysisRule = analysisRule;
    } else {
      _analysisRule = rule.name;
      Registry.ruleRegistry.registerLintRule(rule);    // SEE THIS LINE
    }
    super.setUp();
    newAnalysisOptionsYamlFile(
      testPackageRootPath,
      analysisOptionsContent(experiments: experiments, rules: [_analysisRule]),
    );
  }

CC @srawlins @bwilkerson