[ci] reduce duplication in scripts, clean up containers

[jameslamb]

Contributes to #90

This proposes some miscellaneous cleanup to testing and development scripts.

• reduces duplication in test-integration.R
• passes --rm to all docker run calls, to clean up containers when they exit

Notes for reviewers

After a few weeks of active development here, I have a bunch of stranded containers.

docker container ls --all

These take up a little storage space, and are just noise when you're looking for other containers. Adding --rm to every docekr run ensures that they automatically get cleaned up when the containers exit.