test-proxy is not properly handling HTTPS

Well, our wrapper over test-proxy, that is.

We do set the env vars to use the localhost TLS cert, but it seems what we lack is properly redirecting http and https to the right ports like .NET or Go do.

The Rust implementation is pretty much a port (at least the bootstrapping) of Go's, so I need to dig deeper. But .NET actually has separate endpoints for http and https. Maybe we need them too?

The difficult part is that we bind to port 0, which means give me a random port and that has been http. So what is https in that case?

@scbedd recommends passing *at least* `--url https://localhost:0` but if we need both, we could pass that twice - once for each. Probably should, and then the default would use port 5000 for http and 5001 for https. Then we just always manager 2 ports.

Blocking #4344

Start by reading the Rust test-proxy wrapper and its bootstrapping, then compare its HTTP/HTTPS endpoint setup with the Go implementation and the separate endpoints described for .NET. The work is done when both protocols consistently redirect to the correct ports, including the port-0 case, with the intended default ports.

Written by the indexing model from the issue text.