Update C# tools unit test engine to work

Summary: This updates the C# tools unit test engine (the derived version of xUnit.NET test engine that supports code coverage) to work after the updates in D7594.

Test Plan:
Switched the test engine from 'XUnitTestEngine' to 'CSharpToolsTestEngine' and ran `arc unit --everything`.  The results were identical except that code coverage was provided.

Tested on Linux; will update or comment based on whether it works on Windows as well.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7640