Mention the difference between `with TestClient(app) as client` and `client = TestClient(app)`

Add a note on the TestClient section about the behavior difference between those two ways, i.e. lifespan events.

Considering the follow application:

from starlette.applications import Starlette
from starlette.requests import Request
from starlette.responses import Response
from starlette.routing import Route
from starlette.testclient import TestClient


async def startup() -> None:
    print("Startup event!")

async def endpoint(request: Request) -> Response:
    return Response(status_code=204)

app = Starlette(routes=[Route("/", endpoint)], on_startup=[startup])

The following TestClient prints “Startup event!”:

with TestClient(app) as client:
    ...

But the following, doesn’t:

client = TestClient(app)

[adriangb]

@contextlib.asynccontextmanager
async def lifespan(app):
    async with some_async_resource():
        yield

I thought this was the “recommended” way to do lifespans nowdays

[Kludex]

Closed by #1747