[gardening] Mark regression test slow
[gardening] Mark regression test slow

TBR=sstrickl@google.com

Change-Id: I33ea1cd8fdf8607a81749785d06e004e77a686bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/157740
Reviewed-by: Vyacheslav Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk