* Change test ID from `<filename>` to `<language>/<filename>`
* Speed up test collection by about 1 min and total test time by about
5 min in [sample-programs][sample-programs] by caching list of sources
* Modify `download`, `run`, and `test` commands so that `-p`, `-l`, and
`-s` are no longer mutually exclusive
* Add `--parallel` to `download` command to parallelize image downloads
* Add `--parallel` to `test` command to parallelize tests