improve docs for testing dart fix (#97493)

packages/flutter/lib/fix_data.yaml

@@ -9,6 +9,9 @@
 # from other fixes. In a comment, include a link to the PR where the change
 # requiring the fix was made.
 
+# Every fix must be tested. See the flutter/packages/flutter/test_fixes/README.md
+# file for instructions on testing these data driven fixes.
+
 # For documentation about this file format, see
 # https://dart.dev/go/data-driven-fixes.
 

packages/flutter/test_fixes/README.md

@@ -6,6 +6,14 @@ test the [`dart fix` framework](https://dart.dev/tools/dart-fix) refactorings us
 See the flutter/packages/flutter/lib/fix_data.yaml file for the current package:flutter
 data driven fixes.
 
+To run these tests locally, execute this command in the flutter/packages/flutter/test_fixes
+directory.
+```sh
+dart fix --compare-to-golden
+```
+
+For more documentation about this folder, see https://dart.dev/go/data-driven-fixes#test-folder.
+
 ## When making structural changes to this directory
 
 Note that the tests in this directory are also invoked from external repositories.