[skip changelog] Fix contributing guidelines link on tests readme (#784)

Linked to the Markdown file rather than the documentation site page so the link will always point to the matching docs.

Author: per1234

test/README.md

@@ -34,4 +34,4 @@ To run very specific test functions:
 pytest test_lib.py::test_list
 ```
 
-[0]: ../CONTRIBUTING.md
+[0]: ../docs/CONTRIBUTING.md