This PR adds smoke tests using [Smoke
tool](https://github.com/SamirTalwar/smoke) for all the shell tests we
have. One reason for adopting Smoke instead of the previous tool,
`shelltestrunner`, is that tests are declared cleanly and simply using
Smoke Yaml syntax compared to shelltestrunner's syntax.
To add a new smoke test, create a file with the suffix ".smoke.yaml" in
the `tests/smoke` folder. In such a folder, you can also find examples
of how to test the CLI.
