[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_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")
+	}
+}