All postional args for nox tests

[BradyPlanden]

Feature description

Add positional arguments for nox sessions to selectively choose individual or grouped tests to run.

Motivation

To run individual tests, developers need to use pytest directly via: pytest --unit tests/unit/test_costs.py; however, this does not manage the virtualenv or dependency state. As nox does this inherently, offering a solution that shinks the whole test suite to a user selection would be more robust.

Possible implementation

See PyBaMM's recent implementation: https://github.com/pybamm-team/PyBaMM/pull/4334/files

Additional context

No response