Fix coverage for NoseTestEngine

Summary:
In case that the $source_path is `.` or empty, it produces filenames that
look like `./foo.py`, which differential doesn't like.

Test Plan: arc diff, see coverage data.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D7181