[signin][tests] Identity test utils don't use unknown access point by default

The main change is in identity_test_utils.h

ACCESS_POINT_UNKNOWN is not something that is used in production, and
triggers edge cases and errors. In particular it does not count as an
explicit signin for Uno.

This CL uses a more conventional access point, to match what is
happening in production.

As a result, UNO gets enabled in many more tests.
Some of them are testing pre-UNO behavior, and for those tests, the
UNO feature is explicitly disabled.

Change-Id: Idf379dfe307af289e7099b0850c017981c6eab42
Cq-Do-Not-Cancel-Tryjobs: true
Validate-Test-Flakiness: skip
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5300960


Reviewed-by: Victor Vianna <victorvianna@google.com>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1267411}