Add instructions to run specific tests

[seisman]

Description of proposed changes

Runing the full tests using make test is very slow. For most PRs, we may work on only a small
fraction of the codes, and it's common that we only need to run specific tests.

This PR adds instructions for running specific tests.

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.