Add developer how-to for React Testing Library best practices

[brian-smith-tcril]

After discussing openedx/paragon#2619 in the Paragon working group, it was decided that the best place to document best practices for using React Testing Library to test frontend code within the Open edX project would be in a developer how-to.

[sarina]

We have language-specific style guides here: https://docs.openedx.org/en/latest/developers/references/developer_guide/style_guides/index.html it might be easy to start here and gather references before doing a how-to which can be harder to craft