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
Remove debugging output from xUnit.NET test engine

Summary: This just removes the `print "Discovered test .."` code in the xUnit.NET unit test engine.  If users need to diagnose what tests are being debugged, it can be found out with `--trace` and observing what build commands are invoked.

Test Plan: Ran `arc unit --everything` and didn't see the message any more.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7639
Add output-json option to arc feature

Summary:
The current format of arc feature is not very friendly to parsing.  This adds a json output
format.

Test Plan: Run the command and ensure output is valid

Reviewers: lifeihuang, #blessed_reviewers, epriestley

Reviewed By: epriestley

CC: epriestley, aran, Korvin

Differential Revision: https://secure.phabricator.com/D7647
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
Merge branch 'master' of http://github.com/facebook/arcanist