Allow unit test duration to be submitted as a float or an int

Summary: Fixes T8736. It's OK for duration to be something that comes out of JSON as an int, like 1.

Test Plan: Will make @hach-que verify.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, hach-que

Maniphest Tasks: T8736

Differential Revision: https://secure.phabricator.com/D13531