feat(*): add `make test-style` targets

This gives us an opporunity to run code checkers and linters as part
of testing, as we used to on travis-ci.org. By default, unit tests
will check code syntax and style first.
ref(*): clean up Go and Python code for `make test-style`
Merge pull request #1666 from mboersma/1509-make-test-style

feat(*): add `make test-style` targets