Flake8 fstest and gatewaytest (rebased onto develop)


This is the same as gh-2745 but rebased onto develop.

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This PR is part of https://trello.com/c/vnojR45Q/43-flake8-omeropy.

All files under `test/unit/fstest` and `test/integration/gatewaytest` are reviewed to pass `flake8 and re-included in the global `flake8 -v .` check performed by the Travis Python build. To test this PR, minimally,
- check Travis does not fail and explicitely checks the folders above (in the log)
- check the unit tests are still passing in the Travis log
- check the integration tests are unaffected in the Jenkins CI job
Optionally, run the modified unit and integration tests locally.