[iOS][gardener] Flakify testAccessiblityStringForSignedInUserWithoutName

Mark test as flaky.

Bug: 331928746
Change-Id: Ieed7fa077ed87125d18cbbce2ad3f4956ebd0f32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5407612


Auto-Submit: Ginny Huang <ginnyhuang@chromium.org>
Owners-Override: Ginny Huang <ginnyhuang@chromium.org>
Commit-Queue: Ginny Huang <ginnyhuang@chromium.org>
Reviewed-by: Hira Mahmood <hiramahmood@google.com>
Commit-Queue: Hira Mahmood <hiramahmood@google.com>
Cr-Commit-Position: refs/heads/main@{#1280197}