[C++] Add unit test for native_type usage

tests/vector3d_pack.cpp

@@ -0,0 +1,13 @@
+#include "vector3d_pack.h"
+
+#include "native_type_test_generated.h"
+
+namespace flatbuffers {
+  Geometry::Vector3D Pack(const Native::Vector3D& obj) {
+    return Geometry::Vector3D(obj.x, obj.y, obj.z);
+  }
+
+  const Native::Vector3D UnPack(const Geometry::Vector3D& obj) {
+    return Native::Vector3D(obj.x(), obj.y(), obj.z());
+  }
+}