[test] the hello-world example

[szabgab]

See #3263

[yanns]

Does this test really provides value?
In my opinion, a test sending a HTTP request to the app and checking the http response would provide more value.
You can use

axum/examples/compression/src/tests.rs

Lines 15 to 34 in da3539c

	#[tokio::test]
	async fn handle_uncompressed_request_bodies() {
	// Given
	let body = json();
	let compressed_request = http::Request::post("/")
	.header(header::CONTENT_TYPE, "application/json")
	.body(json_body(&body))
	.unwrap();
	// When
	let response = app().oneshot(compressed_request).await.unwrap();
	// Then
	assert_eq!(response.status(), StatusCode::OK);
	assert_json_eq!(json_from_response(response).await, json());
	}

as an example of such test.

[szabgab]

If I am not mistaken for that I'll have to create the app function and move the Router creation there. I think that would be the better way to do this but I was not sure if that is desirable in this most basic "Hello World" example.