Implement backpressure test using embedded channel

[artemredkin]

[fabianfett]

This was done with the recent reimplementation.

async-http-client/Tests/AsyncHTTPClientTests/HTTP1ClientChannelHandlerTests.swift

Line 23 in 2bd8885

func testResponseBackpressure() {

async-http-client/Tests/AsyncHTTPClientTests/HTTP2ClientRequestHandlerTests.swift

Line 23 in 2bd8885

func testResponseBackpressure() {