Updated xUnitTestEngine to support running all tests.

Summary: Unit test engines need to explicitly support the `--everything` option, which I didn't originally implement into xUnitTestEngine.  This updates xUnitTestEngine to support `--everything` and also addresses a few issues when running the tests from a subfolder of the project.

Test Plan: Run `arc unit --everything`.  All tests should run.

Reviewers: Sabre070, jrhodes

Reviewed By: jrhodes

Differential Revision: http://code.redpointsoftware.com.au/D24