Document using Pinia in testing

[obulat]

When using Pinia outside of the components, as in the store tests, we need to use setActivePinia(createPinia()) to prevent the following error:

Error: [🍍]: getActivePinia was called with no active Pinia. Did you forget to install pinia?
	const pinia = createPinia()
	app.use(pinia)
This will fail in production.

I'm unclear of when it's necessary to use setActivePinia. Could you add a blurb in the TESTING_GUIDELINES explaining that (or else open an issue to do it in a separate PR)?

Originally posted by @sarayourfriend in WordPress/openverse-frontend#1040 (comment)