ci: only test v3 tests

[jhamman]

This PR changes our hatch test script in two ways:

1. renames the run script to a more informative test script
2. only runs tests in the v3 directory

TODO:

• [ ] Add unit tests and/or doctests in docstrings
• [ ] Add docstrings and API docs for any new/modified user-facing classes and functions
• [ ] New/modified features documented in docs/tutorial.rst
• [ ] Changes documented in docs/release.rst
• [ ] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)