Fix engine v2 test step flake population

Test step flake considers only steps with prefix `test:` in the step names. To surface the flake data, this CL updates the step name to be consistent with the real test step name.

Change-Id: I495046ccb6e2896492516e5581381c431a34faa8
Bug: https://github.com/flutter/flutter/issues/138007
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/52180
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Keyong Han <keyonghan@google.com>
diff --git a/recipes/engine_v2/builder.py b/recipes/engine_v2/builder.py
index bc960ed..297eb52 100644
--- a/recipes/engine_v2/builder.py
+++ b/recipes/engine_v2/builder.py
@@ -140,7 +140,7 @@
           api.retry.wrap(
               run_test,
               max_attempts=test.get('max_attempts', 3),
-              step_name=test.get('name')
+              step_name=step_name
           )
       finally:
         api.logs_util.upload_logs(test.get('name'))