feat: add test to reproduce #605

[jonaskinnvall]

What:

This fixes a bug that assumes that tests don't run in SSR environments.

Why:

With frameworks like Next.js and Remix rendering components on the server, it's important that this library can mimic SSR environments as closely as possible.

How:

First by adding a test that reproduces the issue and then by moving this logic "into the hydrate function, so we reference document when they want it to render for real." (suggested by mpeyper).

Checklist:

• Documentation updated
• Tests
• Ready to be merged
• Added myself to contributors table

[jonaskinnvall]

L37, “✍🏼 If container is undefined, then we pass null to hydrate since undefined is not a valid container, but null is.”

[jonaskinnvall]

L52-54, “✍🏼 Since act returns a function, it introduces a new scope which means we need to check again that container is not undefined before using it. This means we could remove the outer if (container) { check if we're okay that unmount always returns act”.