[iOS] Enable testNoSignInScreenWhenSigninFromRegularSigninPrompt

The test failed because it didn't find the snackbar. But according to
the video, the snackbar was displayed. This issue must be related to
the snackbar short duration.
The flakiness is not related to iOS 26.

With crrev.com/c/7018369, the snackbar duration is now to 30s during
EGTests. This test should not be flaky anymore.

Fixed: 439982418
Change-Id: I9a1f65dd3b60738bc2e76599fb9eadfa850b79c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7036453
Reviewed-by: Arthur Milchior <arthurmilchior@chromium.org>
Auto-Submit: Jérôme Lebel <jlebel@chromium.org>
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1528880}