http2: add support for net/http.Server.ConnState tracking

This is required to work with the upcoming rewrite of the
httptest.Server's connection tracking in
https://go-review.googlesource.com/#/c/15151/
which (at least as of patchset 7) doesn't forcefully tear
down a StateNew connection during Server.Wait. That might
change.

In any case, this adds support for ConnState, which users would
expect regardless of protocol in a mixed HTTP/1 and HTTP/2
environment.

Change-Id: I124aafec29dda123a018935fa306f465ae99cd97
Reviewed-on: https://go-review.googlesource.com/15913
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/http2/transport_test.go b/http2/transport_test.go
index b7bd629..7b62aa9 100644
--- a/http2/transport_test.go
+++ b/http2/transport_test.go
@@ -41,7 +41,7 @@
 	const body = "sup"
 	st := newServerTester(t, func(w http.ResponseWriter, r *http.Request) {
 		io.WriteString(w, body)
-	})
+	}, optOnlyServer)
 	defer st.Close()
 
 	tr := &Transport{InsecureTLSDial: true}