Functional tests for `wp term` command

Create / list tests to start.

Is there a way to parse the JSON from STDOUT to get the term ID(s) for deleting?

I notice we're all sorts of inconsistent in how we produce output. Some examples:
- `wp post create` returns the post ID in the output
- `wp term create` simply says "Term created" (probably my fault)
- `wp post list` offers a `--ids` argument, where `wp term list` does not