tests: add test for custom cpp file extensions

tests/cpp_ext/module/module.cpp

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2019 Inria
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     tests
+ * @{
+ *
+ * @file
+ * @brief       Sample module C++ class
+ *
+ * @author      Alexandre Abadie <alexandre.abadie@inria.fr>
+ *
+ * @}
+ */
+
+#include <cstdio>
+
+#include "module.hh"
+
+#error "This should not be built"
+
+module_class::module_class() {}
+
+module_class::~module_class() {}
+
+void module_class::print(void)
+{
+    puts("Hello from C++ module");
+}