Improve testing documentation in `CONTRIBUTING.md` (#54904)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/54904
Test Plan: Imported from OSS
Reviewed By: samestep
Differential Revision: D27407009
Pulled By: ansley
fbshipit-source-id: ae69d8387b55f714fd105efe7c4ecbdd69674f65
