net/dns, util/publicdns: extract public DNS mapping into own package (#4405)

This extracts DOH mapping of known public DNS providers in
forwarder.go into its own package, to be consumed by other repos

Signed-off-by: Jenny Zhang <jz@tailscale.com>

net/dns/publicdns/publicdns_test.go

@@ -0,0 +1,41 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package publicdns
+
+import (
+	"testing"
+
+	"inet.af/netaddr"
+)
+
+func TestInit(t *testing.T) {
+	for baseKey, baseSet := range DoHIPsOfBase() {
+		for _, addr := range baseSet {
+			if KnownDoH()[addr] != baseKey {
+				t.Errorf("Expected %v to map to %s, got %s", addr, baseKey, KnownDoH()[addr])
+			}
+		}
+	}
+}
+
+func TestDohV6(t *testing.T) {
+	tests := []struct {
+		in      string
+		firstIP netaddr.IP
+		want    bool
+	}{
+		{"https://cloudflare-dns.com/dns-query", netaddr.MustParseIP("2606:4700:4700::1111"), true},
+		{"https://dns.google/dns-query", netaddr.MustParseIP("2001:4860:4860::8888"), true},
+		{"bogus", netaddr.IP{}, false},
+	}
+	for _, test := range tests {
+		t.Run(test.in, func(t *testing.T) {
+			ip, ok := DoHV6(test.in)
+			if ok != test.want || ip != test.firstIP {
+				t.Errorf("DohV6 got (%v: IPv6 %v) for %v, want (%v: IPv6 %v)", ip, ok, test.in, test.firstIP, test.want)
+			}
+		})
+	}
+}