When running unit tests, ignore any custom task fields

Summary:
If you have maniphest.custom-field-definitions set to include "required" fields, a bunch of tests which create tasks can fail.

To avoid this, reset this config while running tests.

This mechanism should probably be more general (e.g., reset all config by default, only whitelist some config) but just fix this for now since it's a one-liner and doesn't make eventual cleanup any harder.

Test Plan: Ran arc unit, hitting tests that create tasks.

Reviewers: chad, 20after4

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17595