When an empty input file was passed to the dictionary trainer, it would infinite loop. The added test case exposes the bug, and is fixed with this PR.
Add cli-tests to `make test`. This adds a `python3` dependency to `make test`, but not `make check`. We could make this dependency optional by skipping the tests if `python3` is not present.
Adds the new CLI testing platform that I'm proposing. See the added `README.md` for details.