Refactor `generate_package` fixture to avoid repetition in tests

[matt-graham]

Is Your Feature Request Related to a Problem? Please Describe

We repeating (slight variations of) this snippet

https://github.com/UCL-ARC/python-tooling/blob/f104af20dc6eb0c2c49129fbc32163922f32b098/tests/test_package_generation.py#L102-L108

across a lot of tests.

Describe the Solution You'd Like

I think it would be nicer to have generate_package return the path to the directory the package is created in (test_project_dir) based on the passed tmp_path and config["project_name"] and to have a default_package_config fixture to avoid the repetition of the config specification.

Describe Alternatives You've Considered

No response

Additional Context

No response