wp term list: Support for `--ids` argument
Refactor to allow 'ids' be passed to `format_items()` as a formatting argument
Convert `wp user list --ids` to `wp user list --format=ids`

Also, dropped the formatting of the `$output_items`, as a user is already a WP_User object and the formatting is handled within our `format_items()` util
JSON format test for `wp user list`

Also, fix the CSV used in the setup of the scenario as it originally caused the test to fail