[WebAuthFlow] Cleanup browser tests

Simplify some tests by reducing boilerplate code.
Fixed a potential test flakiness by removing the animation of the
infobar.

Bug: 1454772
Change-Id: I567605e581240886f9f617e8b63f99400d1139d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4620391


Reviewed-by: Alex Ilin <alexilin@chromium.org>
Commit-Queue: Ryan Sultanem <rsult@google.com>
Cr-Commit-Position: refs/heads/main@{#1159581}