Making unit tests work under msbuild.

Summary: A few fixes for making things work under Windows / MSBuild.

Test Plan: Apply the patch and run `arc unit --everything`.  If you have the Visual Studio Command Prompt, test under that as well.

Reviewers: Sabre070

Reviewed By: Sabre070

Maniphest Tasks: T39

Differential Revision: http://code.redpointsoftware.com.au/D30