Re-enable fs tests

This PR fixes the OmeroFS tests such that the Travis build can be re-enabled, see https://github.com/joshmoore/openmicroscopy/commit/9b2c81997c9f0c6dbdc0acfcd4d24b3bba0a2848. The tests were not using `pytest` and so this has been fixed at the same time. It also fixes the tests for `flake8` and enables that check on Travis. Finally, it marks several failing tests as failing for fixing in a later PR. See https://trac.openmicroscopy.org/ome/ticket/12566

Not much to test other than that Travis (and Jenkins?) are green.