R1:64ad44cffb24

Always override `auth.email-domains` when running unit tests

Summary: Fixes T9689, Always override `auth.email-domains` when running unit tests

Test Plan:
- Set `auth.email-domains`
- Run `arc unit --everything`.
Observe no errors.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T9689

Differential Revision: https://secure.phabricator.com/D14456