[enrollment] Extract expectations in EnrollmentScreen tests.

Move expectations from Test* methods into test cases.

Add test cases:
* Test automatic manual fallback for rollback flow.
* Test user initiated manual fallback for rollback flow.

Bug: b:332528379
Test: unit_tests --gtest_filter="*EnrollmentScreen*"
Change-Id: I6376a08ca371d2f5d24ffe66cdbc57b6437e73a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5093936


Reviewed-by: Roland Bock <rbock@google.com>
Commit-Queue: Artem Sumaneev <asumaneev@google.com>
Cr-Commit-Position: refs/heads/main@{#1281833}