[enrollment] Add EnrollmentScreen tests for attestation enrollment

Add unit tests for attestation-based enrollment flows and manual
fallbacks.

Add user retry test case for rollback flow.

Bug: b:332528379
Test: unit_tests --gtest_filter="*EnrollmentScreen*"
Change-Id: Id8fc77b4b708cb18388d03ee5985e735703b1a2c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5419238


Reviewed-by: Roland Bock <rbock@google.com>
Commit-Queue: Artem Sumaneev <asumaneev@google.com>
Cr-Commit-Position: refs/heads/main@{#1281943}