[iOS] Sets the snackbar duration to 30s in EGTests

To avoid flakiness in EGTests, this patch adds:
tests_hook::GetSnackbarMessageDuration()
By default the value stays on the screen during 4s
Based on crbug.com/449158371#comment4 EGTests might might need more
than 4s to detect a snackbar.

To avoid no flakiness for EGTests, with this patch, the duration value
is set to 30s.

Few EGTests are updated to work with this new value.

Bug: 449158371
Change-Id: I0122bdef1e47b2b612d070967aad02e3e2d84cdf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7018369
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1527512}