tests/cpp/test_metainfo: Add tests to save and load

tests/cpp/helpers.cc

@@ -0,0 +1,11 @@
+#include "./helpers.h"
+
+std::string TempFileName() {
+  return std::tmpnam(nullptr);
+}
+
+long GetFileSize(const std::string filename) {
+  struct stat st;
+  stat(filename.c_str(), &st);
+  return st.st_size;
+}