Missing documentation in `testing.js`

# Documentation

- **Shescape version**: [2.1.8](https://npmx.dev/package/shescape/v/2.1.8)
- **Operating system**: n/a
- **Shell**: n/a

## Description

Methods in [`testing.js`](https://github.com/ericcornelissen/shescape/blob/8de3ff246b443082a693b1809a65142bf04d7d60/src/testing.js) are not documented.
