Don't ask for confirmation with unsound unit tests

Test Plan:

$ arc diff # with unsound tests
$ arc diff --ignore-unsound-tests # with unsound tests

Reviewers: epriestley, aran

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1515

Differential Revision: https://secure.phabricator.com/D2985