Document how tests should be structured in our repo

[jplhomer]

In CONTRIBUTING.md we should spell out how tests should be structured and written.

Our unwritten rules up to this point:

• Tests should live in a tests folder (NOT __tests__)
• Tests should end with *.test.ts(x) (NOT .spec.tsx(x))
• Tests should be wrapped with an outer describe('<ComponentName>')
• Use @shopify/react-testing library to test React components
• Use factories (faker) instead of hardcoded fixtures

Note that we have older tests that don't adhere to this convention, but we should migrate them over to this new convention.

[frehner]

I'm in the process of adding Vitest to hydrogen-ui, and I'll add the docs in the CONTRIBUTING.md file for h-ui for these things. 👍