docs: add local testing instructions to contributing guide

[Sushma-1706]

Adds repository-specific instructions for running tests locally, including
pytest usage and notes on tests that require external infrastructure.

This addresses feedback from the JOSS review.

Closes #661

[JoeZiminski]

Hi @Sushma-1706, thanks for this! We can actually add this to the existing contributing guide (you can see on the website here. In the code, this is here and so we can merge this CONTRIBUTING.MD with that page. You can see on the website how to build the docs locally.

Some of this page duplicates what we have, so I think we can move the sections:

1. installing test dependencies (btw we use dev instead of test)
2. Running the full test suite
3. Running a subset of tests
4. Tests requiring additional infrastructure

to go between "Formatting and pre-commit hooks" and "Contributing documentation" in the existing documentation. let me know if you have any questions!