Add `make test` target that runs flake8 etc

This copes the basic `Makefile` from syncstorage to provide a `make test` target, which runs `flake8` in addition to the usual testsuite.  I've found this pretty useful in the past, am open to a non-makefile-based solution but I think we should have an automated flake8 run in there somewhere.

Also fixes all the flake reported by `flake8`, which included at least one actual typo-related bug.