A note about testing in our docs

[oddvernes]

It was suggested we should document how to test ds components, considering the more async, non-react nature of some of our logic after the web release

[mrosvik]

Add information about this on https://designsystemet.no/no/fundamentals/code/setup

[mrosvik]

Inspiration from Punkt: https://punkt.oslo.kommune.no/latest/komponenter-og-maler/komponenter/alert/#implementasjon-i-kode:~:text=Kopier%20kode-,Testing,-Elements%3A%20Dersom%20du