fix(netxlite): improve TLS auto-configuration (#409)

Auto-configure every relevant TLS field as close as possible to
where it's actually used.

As a side effect, add support for mocking the creation of a TLS
connection, which should possibly be useful for uTLS?

Work that is part of https://github.com/ooni/probe/issues/1505

internal/netxmocks/quic_test.go

@@ -1,9 +1,13 @@
 package netxmocks
 
 import (
+	"context"
+	"crypto/tls"
 	"errors"
 	"net"
 	"testing"
+
+	"github.com/lucas-clemente/quic-go"
 )
 
 func TestQUICListenerListen(t *testing.T) {
@@ -21,3 +25,22 @@ func TestQUICListenerListen(t *testing.T) {
 		t.Fatal("expected nil conn here")
 	}
 }
+
+func TestQUICContextDialerDialContext(t *testing.T) {
+	expected := errors.New("mocked error")
+	qcd := &QUICContextDialer{
+		MockDialContext: func(ctx context.Context, network string, address string, tlsConfig *tls.Config, quicConfig *quic.Config) (quic.EarlySession, error) {
+			return nil, expected
+		},
+	}
+	ctx := context.Background()
+	tlsConfig := &tls.Config{}
+	quicConfig := &quic.Config{}
+	sess, err := qcd.DialContext(ctx, "udp", "dns.google:443", tlsConfig, quicConfig)
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected")
+	}
+	if sess != nil {
+		t.Fatal("expected nil session")
+	}
+}