Add "Testing" section to the "CONTRIBUTING.md" file

[kmgalanakis]

Description of the Change

Add a Testing section in the CONTRIBUTING.md file.

Changelog Entry

Added - "Testing" section in the "CONTRIBUTING.md" file.

Credits

Props @kmgalanakis

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [x] All new and existing tests pass.