net/dns/resolver: make DoH dialer use existing dnscache happy eyeball dialer

Simplify the ability to reason about the DoH dialing code by reusing the
dnscache's dialer we already have.

Also, reduce the scope of the "ip" variable we don't want to close over.

This necessarily adds a new field to dnscache.Resolver:
SingleHostStaticResult, for when the caller already knows the IPs to be
returned.

Change-Id: I9f2aef7926f649137a5a3e63eebad6a3fffa48c0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolver/forwarder_test.go

@@ -5,6 +5,7 @@
 package resolver
 
 import (
+	"flag"
 	"fmt"
 	"net"
 	"reflect"
@@ -169,6 +170,25 @@ func TestMaxDoHInFlight(t *testing.T) {
 	}
 }
 
+var testDNS = flag.Bool("test-dns", false, "run tests that require a working DNS server")
+
+func TestGetKnownDoHClientForProvider(t *testing.T) {
+	var fwd forwarder
+	c, ok := fwd.getKnownDoHClientForProvider("https://dns.google/dns-query")
+	if !ok {
+		t.Fatal("not found")
+	}
+	if !*testDNS {
+		t.Skip("skipping without --test-dns")
+	}
+	res, err := c.Head("https://dns.google/")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer res.Body.Close()
+	t.Logf("Got: %+v", res)
+}
+
 func BenchmarkNameFromQuery(b *testing.B) {
 	builder := dns.NewBuilder(nil, dns.Header{})
 	builder.StartQuestions()