Don't show detailed unittest result box if all tests passed

Summary:
When all unit tests pass, a box appears between the unit test results and lint
status (for test failures to go in). This checks if there's anything to put
in that div/ul before putting it on the page.

Test Plan:
Loaded a revision with unit tests OK and saw no box. Loaded a revision with
failing unittests, and saw the same box from before.

Reviewers: tuomaspelkonen, epriestley

Reviewed By: epriestley

CC: jungejason, aran, nh, epriestley

Differential Revision: https://secure.phabricator.com/D1295