Convert some simpler feature tests to controller and view tests to improve test speed

[mitchellhenke]

🛠 Summary of changes

spec/features/users/sign_in_spec.rb is the test file that takes the most time and knapsack only distributes by spec files. The sign_in_spec file is often the only file run in its parallel test runner, and it is still the longest running one. This PR attempts to reduce the time it takes to run this file by moving some of the simpler tests out to more efficient test types and removing one duplicative one.

{
 "spec/features/users/sign_in_spec.rb"=>365.684857889,
# ⬆️
 "spec/features/accessibility/idv_pages_spec.rb"=>311.30431135,
 "spec/features/accessibility/user_pages_spec.rb"=>284.783216708,
 "spec/features/idv/in_person_spec.rb"=>256.036439207,
 "spec/features/idv/steps/phone_step_spec.rb"=>212.701576452,
 "spec/features/remember_device/sp_expiration_spec.rb"=>210.965194528,
 "spec/features/openid_connect/openid_connect_spec.rb"=>175.398915327,
 "spec/features/users/sign_up_spec.rb"=>147.282360199,
 "spec/features/idv/sp_handoff_spec.rb"=>123.087861688,
 "spec/features/idv/doc_auth/verify_info_step_spec.rb"=>119.300631601,
# ...
}

[zachmargolis]

LGTM! some small suggestions about stubbing