Add server.url property in tests #288

We are defining the URL to the uvicorn server that's running during our tests in lots of place, e.g.:

# tests/client/test_async_client.py

async def test_get(server, backend):
    url = "http://127.0.0.1:8000/"
    async with httpx.AsyncClient(backend=backend) as client:
        response = await client.get(url)
    # ...

async def test_raise_for_status(server, backend):
    async with httpx.AsyncClient(backend=backend) as client:
        for status_code in (200, 400, 404, 500, 505):
            response = await client.request(
                "GET", f"http://127.0.0.1:8000/status/{status_code}"
            )
        # ...

As suggested by @sethmlarson in #276 (comment), it would be better to use a URL property on the server provided by the server and https_server fixtures instead. This way, we can change the root URL in one place (close to wherre the server is actually defined), while being able to build child URLs using server.url.copy_with(path="/echo").

This can probably be done by creating and using a uvicorn Server subclass with the URL instance set as a class attribute.