Prepare test image

A script `hack/prepare_tests.sh` that builds a test image that
- installs docker test dependencies
- pulls `runtime_test.go`'s `docker-test-image` into the correct graph location

The test suite can then use this image as its basis. I've wrapped this up in `hack/run_tests.sh`

This makes running the test suite much faster, since it doesn't have to do all the preparation each time.

The existing test running approach should still work.