Fix issues in C# unit test engine

Summary: This fixes a few issues in the C# unit test engine.  It fixes tests sitting in subdirectories not being tested correctly (the location of both the test assembly and the results file would be wrong).  It also fixes a very strange issue where xUnit.NET seems to not output the resulting XML file when it executes; in this case we just retry running the test until the XML file appears after completion (and eventually it works).

Test Plan: Ran `arc unit --everything` and `arc unit --everything --no-coverage` and verified that it's all reliably working.

Reviewers: epriestley, #blessed_reviewers, hach-que

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7594