I added a first test but am a little lost in how and where I could test the integrated functions. I'd love for this to get merged but would also appreciate help on how to write more tests here.

I think you can put it in packages/vite/src/node/__tests__/dev.spec.ts with something like

test('check', () => {
  expect.assertions(1)
  const { promise, resolve } = promiseWithResolvers<void>()
  const server = await createServer({
    root: __dirname,
    logLevel: 'error',
    server: {
      port: 9400, // make sure the port is unique
      ws: false,
    },
    plugins: [/* add plugin that checks server.resolvedUrls here */] 
  })
  await server.listen()
  await promise
})

It needs to wait for the configureServer hook to be called so using promiseWithResolvers is needed.
https://github.com/vitejs/vite/pull/19936/files#diff-0244b1dfcfa697a921ae7858a6beb979114a8c59b478ff360e72fdeb82b0b82cR226-R247

I added a first test but am a little lost in how and where I could test the integrated functions. I'd love for this to get merged but would also appreciate help on how to write more tests here.

I think you can put it in packages/vite/src/node/__tests__/dev.spec.ts with something like

test('check', () => {
  expect.assertions(1)
  const { promise, resolve } = promiseWithResolvers<void>()
  const server = await createServer({
    root: __dirname,
    logLevel: 'error',
    server: {
      port: 9400, // make sure the port is unique
      ws: false,
    },
    plugins: [/* add plugin that checks server.resolvedUrls here */] 
  })
  await server.listen()
  await promise
})

It needs to wait for the configureServer hook to be called so using promiseWithResolvers is needed. https://github.com/vitejs/vite/pull/19936/files#diff-0244b1dfcfa697a921ae7858a6beb979114a8c59b478ff360e72fdeb82b0b82cR226-R247

Thanks for the suggestion. That worked out very good. I added a new suite to the dev.spec.ts that fails without this patch and is green with this patch.

Has the Windows test fail anything to do with these changes? Is there something I can do here?

It's probably a flaky fail. I've retriggered the CI 👍