Improve reliability of flakey countdown spec

[aduth]

🛠 Summary of changes

Rewrites parts of a sign-in spec to improve reliability. This had previously been attempted for improvement in #7466, but build failures still occur (example).

This rewrite also has a few improvements:

• Passes regardless of locale
• Passes regardless of text changes to datetime/en.yml string data
• Passes regardless of minutes being traversed (Edit: Removed in 6e5221b, see Improve reliability of flakey countdown spec #7512 (comment))
• Passes regardless of config changes to session_timeout_in_minutes

📜 Testing Plan

rspec spec/features/users/sign_in_spec.rb:269

[aduth]

This rewrite also has a few improvements:

I suppose an alternative for many of these could have been to stub out all of the relevant values (config and locale string data), which may have made the spec stay a little easier to read. Not stressing it too much, but happy to revisit if there's any strong reactions to it.