docs: document how to run the test suite locally #430

Problem

When working on a test contribution, I had to inspect the repository's test configuration and CI workflow to determine how to run the same test suite locally.

The project has an established pytest-based test suite, but as a new contributor, I initially had to investigate the repository configuration to understand the expected local testing workflow.

Proposed documentation

Add a short section to the contributor/development documentation explaining:

• how to install the test dependencies
• how to run the complete test suite
• how to run a specific test file
• how to run a focused test using pytest's -k option
• why some tests may be skipped
• where contributors can find the CI test configuration

Who this is for

This is intended for new contributors making their first test-related change who need to verify their changes locally before opening a pull request.

Why I am proposing this

This is based on a real point where I was confused during my test contribution. I had to inspect the project's test configuration and CI workflow to understand how local pytest execution related to the automated checks on a pull request.