all: use of nettest, remove internal/nettest

This change uses the nettest package where possible and removes the
internal/nettest package.

Change-Id: I5615a3ab7957183fecea6b5646df99dbb7c186e2
Reviewed-on: https://go-review.googlesource.com/c/net/+/123057
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
diff --git a/ipv6/helper_posix_test.go b/ipv6/helper_posix_test.go
new file mode 100644
index 0000000..cc85cea
--- /dev/null
+++ b/ipv6/helper_posix_test.go
@@ -0,0 +1,31 @@
+// Copyright 2014 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.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris windows
+
+package ipv6_test
+
+import (
+	"os"
+	"syscall"
+)
+
+func protocolNotSupported(err error) bool {
+	switch err := err.(type) {
+	case syscall.Errno:
+		switch err {
+		case syscall.EPROTONOSUPPORT, syscall.ENOPROTOOPT:
+			return true
+		}
+	case *os.SyscallError:
+		switch err := err.Err.(type) {
+		case syscall.Errno:
+			switch err {
+			case syscall.EPROTONOSUPPORT, syscall.ENOPROTOOPT:
+				return true
+			}
+		}
+	}
+	return false
+}