Add support for testing

[mintydev789]

It's currently difficult to test server functions created with this library (or even without it, I imagine). It would be nice if there was some support for mocking them out of the box, just like with the nuqs library. When I try to run a server function wrapped in next-safe-action using Vitest, I get the following error due to it not being able to access the headers:

Action error: `headers` was called outside a request scope. Read more: https://nextjs.org/docs/messages/next-dynamic-api-wrong-context

[mintydev789]

I just realized that it was my auth step that was accessing the headers, which I was able to circumvent by checking if I'm in a testing environment and just always skipping auth in that case