Allow overriding headless test URL

[lynn]

I have wasm code that uses web-sys to make cross-origin requests with cookies. To test this interactively, I navigate to http://localhost.mydomain.org:8000 (which I've set up to resolve to 127.0.0.1) instead of http://localhost:8000.

This PR would allow me to do the same for headless tests. I imagine it's also useful in combination with #3314.

[daxpedda]

I don't think we need a new environment variable here, lets reuse WASM_BINDGEN_TEST_ADDRESS.

Please add an entry in the changelog as well.

[lynn]

I've made the requested changes. However, it feels a bit wrong to me to reuse that variable. It seems like WASM_BINDGEN_TEST_ADDRESS is an address specifying where to bind the test server, and this variable is a URL specifying how to reach it. In Rust terms, one is a SocketAddr and the other is a Url. I admit this distinction may not matter so much, though.

[daxpedda]

Thank you!

[daxpedda]

Suggested change

	let url =
	std::env::var("WASM_BINDGEN_TEST_ADDRESS").unwrap_or_else(|_| format!("http://{}", server));
	let url = format!("http://{}", std::env::var("WASM_BINDGEN_TEST_ADDRESS").unwrap_or(server));

I was briefly thinking that this might be a good idea, but maybe there is a use case out there for https?