Improve tests for the enrollment state fetcher

Adds tests for failing state retrieval on Flex devices with enrollment
token and no PSM state. Also improves the test for failure to retrieve
state keys to also verify that retries are limited.

Change-Id: I0409cdecfc91c0f424000d4fa88a57b0b662d632
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6728274
Reviewed-by: Roland Bock <rbock@google.com>
Auto-Submit: Sergiy Belozorov <sergiyb@chromium.org>
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1485725}