test: add xfail test for non-file multipart request bodies

[adriangb]

As per Discord discussion.

Here's the Starlette equivalent for context:

from starlette import applications, requests, responses, routing, testclient


def test_request_body_multi_part_mixed_field_content_types_starlette() -> None:
    async def test_method(request: requests.Request) -> responses.Response:
        form = await request.form()
        assert await form["file"].read() == b"file data"
        assert form["tags"] == "tags=1&tags=2"
        return responses.Response(status_code=201)
    
    app = applications.Starlette(routes=[routing.Route("/", test_method, methods=["POST"])])

    with testclient.TestClient(app) as client:
        response = client.post(
            "/",
            files=[
                ("file", ("somefile.txt", b"file data")),
                ("tags", (None, b"tags=1&tags=2", "application/x-www-form-urlencoded")),
            ],
        )
        assert response.status_code == HTTP_201_CREATED

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