Add pytest tests for CLI API

[matthewfeickert]

In PR #349, @matthewfeickert forgot to add in the CLI API entrypoint and that had to be patched in PR #350. 😬

It would be good to add tests of the CLI API behavior to ensure that things are working as expected. This can be done using pytest-console-scripts and click.testing.CliRunner. c.f. pyhf's tests for examples of using this.

[BenGalewsky]

Completed via #536