Ensure that between stop() and start() the http url doesn't change

[b1zzu]

We should ensure that between server.stop() and server.start() the url doesn't change

const server = TestxServer(`blablabla`);

await server.start()
const firstUrl = server.url()

await server.stop()
await server.start()
const secondUrl = server.url()

assert(firstUrl === secondUrl)

More details here: #45 (comment)