Fix AccountsTableTestCase with kReplaceSyncPromosWithSignInPromos

There were two main issues with these tests:
1) Many tests verified something in the accounts table view (as the
   name suggests), but it now takes one more tap to get there. This CL
   introduces a helper method to get to that view, which works both
   with and without kReplaceSyncPromosWithSignInPromos enabled.
2) Some of the tests just don't apply anymore, e.g. because they test
   something about the signout confirmation dialog, which doesn't
   exist anymore with kReplaceSyncPromosWithSignInPromos enabled. For
   these tests, kReplaceSyncPromosWithSignInPromos gets explicitly
   disabled.

Bug: 1462156
Change-Id: I165161c60e0016a12d325afbfc72a350af5963e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4817368
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Jérôme Lebel <jlebel@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1189407}