Enable running tests under Docker. This makes it easier to spin up an
environment with all dependencies (especially the multiple versions of
openssl and gnutls needed).
* tests/docker/xenial/Dockerfile: Definition for the docker image,
including local builds for openssl and gnutls.
* tests/scripts/docker_env.sh: New helper script that creates the Docker
image and has a function to run a command in the Docker container.
* tests/docker/all-in-docker.sh: Wrapper for all.sh under Docker.
* tests/docker/basic-in-docker.sh: Script that runs the same commands as
.travis.yml, in Docker.
* tests/ssl-opt-in-docker.sh: Wrapper to run ssl-opt.sh in Docker.
* tests/compat-in-docker.sh: Wrapper to run compat.sh in Docker.
* tests/make-in-docker.sh: Wrapper to run make in Docker.
Change-Id: Ie092b1deed24c24c3859754535589523ce1d0a58
