refactor: move quicdialing base functionality to netxlite (#406)

Part of https://github.com/ooni/probe/issues/1505

internal/netxmocks/quic.go

@@ -0,0 +1,13 @@
+package netxmocks
+
+import "net"
+
+// QUICListener is a mockable netxlite.QUICListener.
+type QUICListener struct {
+	MockListen func(addr *net.UDPAddr) (net.PacketConn, error)
+}
+
+// Listen calls MockListen.
+func (ql *QUICListener) Listen(addr *net.UDPAddr) (net.PacketConn, error) {
+	return ql.MockListen(addr)
+}

internal/netxmocks/quic_test.go

@@ -0,0 +1,23 @@
+package netxmocks
+
+import (
+	"errors"
+	"net"
+	"testing"
+)
+
+func TestQUICListenerListen(t *testing.T) {
+	expected := errors.New("mocked error")
+	ql := &QUICListener{
+		MockListen: func(addr *net.UDPAddr) (net.PacketConn, error) {
+			return nil, expected
+		},
+	}
+	pconn, err := ql.Listen(&net.UDPAddr{})
+	if !errors.Is(err, expected) {
+		t.Fatal("not the error we expected", expected)
+	}
+	if pconn != nil {
+		t.Fatal("expected nil conn here")
+	}
+}