[iOS] Add EGTests for Reading List when re-sign-in with the promo

Adds EGTests for the case the user re-sign-in with the Reading List
promo, checking that:
- the signed-in identity is correct if another account is used for the second sign-in;
- if a items is removed before sign-out, it does not appear after
re-sign-in with the same account;
- if an item is marked as read, it remains read after re-sign-in with
the same account.

Due to the bug of multiple refresh after an item is moved, it's needed
to add a delay after the item is moved before checking the UI
components, which could be hopefully removed once crbug.com/1445875 will
be fixed.

Bug: 1450837, 1445875
Change-Id: I4882073871bf796bed596f9fc2d1612ed60b9278
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4678642
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Menghan Yang <myuu@google.com>
Cr-Commit-Position: refs/heads/main@{#1172330}