Don't load extensions when running unit tests
Summary: Fixes T15519
Test Plan:
- Run `arc unit` with a bogus extension in load-libraries and see it get slightly farther than before (it still fails because it tries to run the storage-creation script which loads extensions)
- With D26704 also installed run `arc unit` and see it only try to extract core libraries.
Reviewers: O1 Blessed Committers, avivey
Reviewed By: O1 Blessed Committers, avivey
Subscribers: avivey, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15519
Differential Revision: https://we.phorge.it/D26707