Setup test environment

We need to setup the test environment for solstice-components.

We cannot rely on JSDOM, the headless browser / polyfill library which we used for solstice-assets, because of the fact that we are using Shadow DOM with our web components.

What I'm using instead is Web Test Runner and Playwright.

I will also use the Open Web Components library for test utils to help with web component tests.

These libraries were used in the test examples of Lit's docs.