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