test(cli): add cli code coverage discovery

This will enable code coverage information to be displayed for
src/cli.js.

The `nyc` package will be required since `child_process` functions
such as `exec` will be used for testing and `nyc` can detect code
coverage for spawned processes and jest cannot at the moment, see
(jestjs/jest#3190).

The `esm` package will be used to load the `cli.js` module during
testing since it's written using ECMAScript and that is not
supported by node`child_process` executing a script.