vendor: update golang.org/x/net/internal/nettest from upstream

Updates to x/net git rev 640f462 for CL 109995.

Change-Id: I081d4c2265c576d61833bdf31ef8e388588053d3
Reviewed-on: https://go-review.googlesource.com/111555Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/vendor/golang_org/x/net/internal/nettest/helper_stub.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build nacl plan9
+// +build js,wasm nacl plan9
 
 package nettest
 

src/vendor/golang_org/x/net/internal/nettest/stack.go

@@ -64,7 +64,7 @@ func TestableNetwork(network string) bool {
 	switch network {
 	case "unix", "unixgram":
 		switch runtime.GOOS {
-		case "android", "nacl", "plan9", "windows":
+		case "android", "js", "nacl", "plan9", "windows":
 			return false
 		}
 		if runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64") {
@@ -72,7 +72,7 @@ func TestableNetwork(network string) bool {
 		}
 	case "unixpacket":
 		switch runtime.GOOS {
-		case "android", "darwin", "freebsd", "nacl", "plan9", "windows":
+		case "android", "darwin", "freebsd", "js", "nacl", "plan9", "windows":
 			return false
 		case "netbsd":
 			// It passes on amd64 at least. 386 fails (Issue 22927). arm is unknown.