Rewrite System.Net.Http.Json functional tests to use a custom HttpMessageHandler

[akoeplinger]

All of the tests start a socket-based loopback server which doesn't work on WebAssembly.
We can do the tests using a custom HttpMessageHandler instead of the loopback server as well.

[ghost]

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[eerhardt]

System.Net.Http.Json is used by the default Blazor WASM template, and I'd assume the majority of Blazor WASM apps that request JSON payloads over Http. Is there any other way we can test the functionality without creating a socket-based loopback server?

[akoeplinger]

@eerhardt I rewrote the tests to remove the dependency on the socket-based server

[mdh1418]

Looks good besides those two parts that weren't clear to me.

[ManickaP]

This will probably collide with #38635.
If, by any miracle, I'm first to merge, I can help with fixing follow up conflicts, just let me know.

[jozkee]

Just one question; otherwise, looks good.

[jozkee]

Why is that you only have to call HandleRequestAsync once now?

[akoeplinger]

@jozkee In the previous implementation we actually had to create four responses from the socket-based loopback server, one for each request since the callback was only invoked once.

Now we invoke the callback for each request so this is no longer necessary.