feat: allow to run e2e tests against existing server

[tobiasdiez]

Allow to pass the url of an already running server to setup and use this as the target instead of building and running a new server. This has two use cases:

1. Run tests against a deployment of your application (i.e. "real" end-to-end tests)
2. During local development, run tests against a already running dev server. This reduces the time to run tests and was wished for in eg https://github.com/nuxt/test-utils/issues/314#issuecomment-1983129269.

Usage:

setup({ endpoint: process.env.TEST_URL })

(or of course just using a hard-coded url)