[ios] Fix flaky test in PushNotificationSettingsUtilTest

Change-Id: Ic6bcdcb3c759dd128d1e946ac860e50119c2c044
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5424914


Commit-Queue: Scott Yoder <scottyoder@google.com>
Reviewed-by: Guillem Perez <guiperez@google.com>
Cr-Commit-Position: refs/heads/main@{#1283590}