[FedCM] Simplify multi account chooser tests

This CL adds the separator between the title and the contents to the
multi account chooser container, so that it is easier to follow the
logic. It also refactors tests by adding a method which makes the
structural checks to the multi accounts container.

Change-Id: I01ea8f519f36ce60959f4faeb8f1f542094a73e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5792207


Commit-Queue: Nicolás Peña <npm@chromium.org>
Reviewed-by: Zachary Tan <tanzachary@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1343017}