[iOS] Disabled flaky SigninCoordinatorTestCase on Simulators

TBR=fernandex@chromium.org

Bug: 1146459
Change-Id: I4b8163481ee473e049cb54504698dc08fbc5a466
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521707


Reviewed-by: Sebastien Lalancette <seblalancette@chromium.org>
Commit-Queue: Sebastien Lalancette <seblalancette@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824912}