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