Cast old duration values for unit tests to float in DifferentialController
Summary: Fixes T10549.
Test Plan: N/A
Reviewers: Blessed Reviewers, avivey, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin
Maniphest Tasks: T10549
Differential Revision: https://secure.phabricator.com/D15452