Improve test infrastructure

[kinnala]

I'd like to run the tests in a Docker container so that I'm able to locally figure out what went wrong in the build. Simultaneously I'd change the test infrastructure to use pytest.

My plan is

• to create /docker_tests/Dockerfile
• push manually the testing container to Dockerhub
• follow instructions at https://docs.travis-ci.com/user/docker/ to run tests inside the container

[gdmcbain]

There are lots of advantages to pytest, yes; I think its worth switching to it from what's available in the standard library.

I also like the idea of containerization. I haven't quite been convinced that Docker is the last word in that, but if it's supported by the continuous integration system then it's worth using, yes.