Try: Fix second MFA reminder flakey spec

[aduth]

🛠 Summary of changes

Attempts to resolve an issue where the "Second MFA Reminder user with multiple mfas who would otherwise be candidate does not prompt the user on sign in" spec occasionally fails in continuous integration.

Example: https://gitlab.login.gov/lg/identity-idp/-/jobs/1371605

It's not entirely clear what causes this, but it's hypothesized that something with the time travel in the existing implementation results in the user arriving at the sign-in page rather than the account page.

The changes here reorganize the test to avoid relying on time helpers, and instead stub the user in a way where the account is old enough to be prompted for the second MFA reminder.

This also has the upside of avoiding magic numbers within the test (travel 6.days), using the default configuration values instead.

Easiest to review with whitespace changes hidden: https://github.com/18F/identity-idp/pull/11109/files?w=1

📜 Testing Plan

Verify that the build passes.