Add instructions to test React apps built with primer/react

[Zenulous]

Recently I've been working on a project that utilizes this component library. We use jest and react-test-renderer to create tests for our frontend. There are issues when running the tests due to CSS definitions:

Solutions for this include importing helpers like https://github.com/primer/react/blob/main/src/utils/test-helpers.tsx, and I think it can be valuable to refer to this in documentation as it took me quite some time to figure out how to properly mock CSS.

[lesliecdubs]

👋🏻 @JoseInTheArena would this be of interest to Web Systems in regards to providing documentation about how to test React apps on the paved path?