Run Behat tests in a Docker container

[danielbachhuber]

From https://github.com/wp-cli/doctor-command/pull/128

I don't keep mysql and some of the required tools installed directly on my machine anymore, so getting the tests to run locally was a big pain point. After pushing a ton of dumb commits to Travis to test things, I ended up throwing together a docker container. This could basically give anybody a one-line command to run tests without having to set anything up, other than Docker (bash bin/test-in-docker.sh)

cc @davegaeddert