Move testlib to internal

internal/testlib/mtglib_antireplay_cache_mock.go

@@ -0,0 +1,11 @@
+package testlib
+
+import "github.com/stretchr/testify/mock"
+
+type MtglibAntiReplayCacheMock struct {
+	mock.Mock
+}
+
+func (m *MtglibAntiReplayCacheMock) SeenBefore(data []byte) bool {
+	return m.Called(data).Bool(0)
+}