Add gRPC tests for trailers-only

[jhump]

This mirrors the gRPC-Web test cases that also handle different forms of trailers in responses.

These weren't originally included because I didn't think they would work: I thought that the "net/http" server would encode these on the wire in HTTP/2 using a HEADER frame (without END_STREAM flag) followed by an empty DATA frame (with END_STREAM flag). But the gRPC spec (and Google's implementations) require a trailers-only response in HTTP/2 be a single HEADER frame with END_STREAM flag set.

However, I was mistaken: the Go "net/http" server does exactly what we need it to. It would only use two frames (i.e. a second, empty DATA frame) if the handler flushes the http.ResponseWriter before it returns. So I didn't have to actually touch the reference server for this to work. 🎉

[jhump]

cc @timostamm