Add diff-cover output when coverage is collected (#2631)
Add diff-cover output when coverage is collected (#2631)

* Add diff-cover output when coverage is collected

This commit adds an additional output stage to the python 3.6 job, which
is used for collecting coverage, that runs the diff-cover tool to show
which parts of the diff have test coverage. This is useful for both
reviewers and the commit author to see if the changes being made in a PR
are actually being exercised by the testing we run.

* Fix typo in coverage.xml filename

QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py