[signin] Test OnAuthErrorUpdated event after adding new account

This CL adds tests documenting the current order of ProfileOAuth2TokenServiceObserver events fired after adding a new
account to PO2TS and updating an existing valid account.

There is no unified unit test file that would test delegates from
all platforms, so this CL adds a new
OnAuthErrorChangedAfterUpdatingCredentials test to each delegate
unittest file.

This CL also unifies multiple mocks of
ProfileOAuth2TokenServiceObserver scattered across unitttest files into
a common MockProfileOAuth2TokenServiceObserver. In addition to code deduplication, this also removes the risk of ODR-violation errors --
there is currently two classes called MockOAuth2TokenServiceObserver
that are exported from their translation units. A modification to one
of them would currently lead to undefined behavior.

Bug: 366067964
Change-Id: Ib87a64be0935d84bc88c1565471e92bafe839599
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5854723
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1355076}