Fix unit test coverage for `NoseTestEngine`.

Summary:
I noticed that code coverage wasn't showing in Differential for
some repositories that we are using with Phabricator.

`arc unit` would should unit test coverage, but the paths were messy
(for example, `.//foo/bar.py` instead of `foo/bar.py`). As a result,
the code coverage info wasn't recognised as being for the correct
module.

I'm not sure why this logic is the way that it is... perhaps this is to
do with an older version of `nose` (I am using v1.3.0).

Test Plan:
I created a diff for an internal repository that we have, and
observed that code coverage was displayed in Differential.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran, seporaitis, avive, dctrwatson, zeeg

Differential Revision: https://secure.phabricator.com/D8433