[HTTP-Pipelining] Test pipelined requests maintain order

[t089]

This adds a simple test that asserts that pipelined requests are always responded to in the correct order if supportPipelining: true is set.

[t089]

I've added a test that triggers the assertion in debug builds even if pipelining is enabled.

http/Sources/HTTP/Responder/HTTPServer.swift

Line 150 in 939d1f9

debugOnly { assert(!self.awaitingReponse, "Pipelined HTTP request detected. Enable pipelining to prevent this error.")}