Create `libtmux.testing` module as a pytest plugin



Implement it like this: 

```python
pytest_plugins = ("libtmux.testing.fixtures",)
```

[example](https://github.com/chrisjsewell/sphinx-pytest/blob/main/src/sphinx_pytest/plugin.py#L18) from https://github.com/chrisjsewell/sphinx-pytest, via [`sphinx.tests`](https://github.com/sphinx-doc/sphinx/tree/5.x/tests)