[cxx]: simple spi master class

* spi cxx unit test (CATCH-based, on host)
* added portmacro.h to driver mocking
* added simple testing app to write/read SPI,
  using an MPU9250

examples/cxx/experimental/experimental_cpp_component/test/unity_cxx.hpp

@@ -1,3 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2021 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
 #pragma once
 
 #include "unity.h"