Add fixtures and integrate with pytest

[benhoyt]

These would live either in a _fixtures.py imported into __init__.py, or perhaps better, a separate jubilant.fixtures module.

We at least need a fixture to create a model with a random model name and yield the jubilant.Juju instance. Not sure what other fixtures we should start with. Look at Pietro's conftest.py for inspiration.

Do we also need pytest command-line options, similar to Pietro's? IMO let's keep it more minimal for v1.