net/http: add more tests of transport connection pool

Add a variety of addtional tests exercising client connection pooling,
in particular HTTP/2 connection behavior.

Change-Id: I7609d36db5865f1b95c903cfadb0c3233e046c09
Reviewed-on: https://go-review.googlesource.com/c/go/+/615896
Reviewed-by: Nicholas Husin <[email protected]>
Reviewed-by: Nicholas Husin <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Damien Neil <[email protected]>

Author: neild

src/net/http/clientserver_test.go

@@ -207,6 +207,8 @@ func newClientServerTest(t testing.TB, mode testMode, h Handler, opts ...any) *c
 			transportFuncs = append(transportFuncs, opt)
 		case func(*httptest.Server):
 			opt(cst.ts)
+		case func(*Server):
+			opt(cst.ts.Config)
 		default:
 			t.Fatalf("unhandled option type %T", opt)
 		}

src/net/http/http2_test.go

@@ -0,0 +1,12 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build !nethttpomithttp2
+
+package http
+
+func init() {
+	// Disable HTTP/2 internal channel pooling which interferes with synctest.
+	http2inTests = true
+}