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