Speed up loading of diffs with a lot of unit test failures

Summary:
We've been having trouble with viewing diffs timing out when there's a lot of unit test failures. It was caused by formatting userdata for every single failure. The expensive part of this was actually creating the engine for every result, so moved the construction outside of the loop.

Diffs that timed out (2 min) loading before load in around 6 seconds now.

Test Plan: Loaded diffs that used to time out. Verified that details still looked right when Show Full Unit Test Results Is Clicked.

Reviewers: epriestley, keegancsmith, lifeihuang, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, andrewjcg

Differential Revision: https://secure.phabricator.com/D7581