[SAML] Proper SAML setup in lock screen online reauth tests

LockScreenReauthHandler used two test-only constants:
force_saml_redirect_for_testing_ and  kIdpTestingDomain. This prevented
testing some code paths. This cl adds additional post-login steps in the
tests, allowing to remove those test-only constants and to rely on the
same code as in production.

This unblocks testing of Gaia reauth endpoint in many tests in
saml_lockscreen_browsertest.cc. Affected tests are now moved to a
dedicated suite SamlUnlockTest and parameterized by the feature flag
GaiaReauthEndpoint.

Bug: b:318077327
Change-Id: I1082a45a3e94a5c1defe35b8510d657817d2f55a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5384257
Reviewed-by: Mohammed Abdon <mohammedabdon@chromium.org>
Reviewed-by: Elie Maamari <emaamari@google.com>
Commit-Queue: Andrey Davydov <andreydav@google.com>
Cr-Commit-Position: refs/heads/main@{#1277555}