[forwardport] fix: avoid http3 for dns.google and www.google.com (#593) (#594)

This commit forward ports dedd84fa7ecb09f718f6b1a9c83999cb37b34dfa.

Original commit message:

- - -

This diff changes code the release/3.11 branch to ensure we're not using dns.google and www.google.com over HTTP3. As documented in https://github.com/ooni/probe/issues/1873, since this morning (approx) these services do not support HTTP3 anymore. (I didn't bother with checking whether this issue affects _other_ Google services; I just limited my analysis to the services that we were using as part of testing.)

This patch WILL require forward porting to the master branch.

internal/netxlite/quictesting/quictesting.go

@@ -0,0 +1,38 @@
+// Package quictesting contains code useful to test QUIC.
+package quictesting
+
+import (
+	"context"
+	"net"
+	"strings"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
+)
+
+// Domain is the the domain we should be testing using QUIC.
+const Domain = "www.cloudflare.com"
+
+// Address is the address we should be testing using QUIC.
+var Address string
+
+// Endpoint returns the endpoint to test using QUIC by combining
+// the Address variable with the given port.
+func Endpoint(port string) string {
+	return net.JoinHostPort(Address, port)
+}
+
+func init() {
+	const timeout = 10 * time.Second
+	ctx, cancel := context.WithTimeout(context.Background(), timeout)
+	defer cancel()
+	reso := &net.Resolver{}
+	addrs, err := reso.LookupHost(ctx, Domain)
+	runtimex.PanicOnError(err, "reso.LookupHost failed")
+	for _, addr := range addrs {
+		if !strings.Contains(addr, ":") {
+			Address = addr
+			break
+		}
+	}
+}

internal/netxlite/quictesting/quictesting_test.go

@@ -0,0 +1,20 @@
+package quictesting
+
+import (
+	"net"
+	"testing"
+)
+
+func TestWorksAsIntended(t *testing.T) {
+	epnt := Endpoint("443")
+	addr, port, err := net.SplitHostPort(epnt)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if addr != Address {
+		t.Fatal("invalid addr")
+	}
+	if port != "443" {
+		t.Fatal("invalid port")
+	}
+}