Updating unit test engine.

Summary: This updates the unit test engine to cleanup result XML files after it's run, which prevents the runner from failing and the engine then showing old results.

Test Plan:
  - `git clean -xdf`
  - `git reset --hard HEAD`
  - `git submodule foreach --recursive git clean -xdf`
  - `git submodule foreach --recursive git reset --hard HEAD`
  - `git submodule update --init --recursive`
  - `arc unit --everything`

The unit tests should all pass.

Reviewers: Sabre070

Reviewed By: Sabre070

Maniphest Tasks: T39

Differential Revision: http://code.redpointsoftware.com.au/D27