Before this commit we used to run the entire unit test suite
in the presence of a failure. Since we currently rely a lot
on the state of the filesystem etc. the end report was most
of the time showing spurious failures that were a consequence
of the first failing test.
This PR makes unit tests exit at the first failing test
Also, pin codecov at v4.5.4 (last one supporting Python 2.6)
f80491826b