tests: Fix versions test pep8 naming errors
tests: Add noqa markers to custom assert helpers
tests: Add noqa markers for setUp/tearDown
tests: Fix MPD tests helper names
tests: Fix outstanding flake8 errors in tests
tests: Update tox flake8 settings

Run with show source so we can see the context in travis errors. Add stats for
number of errors. And limit ourselves to mopidy and tests as I happen to have
a tmp folder with non-conforming proof of concept code that I don't want this
to check.