UnPackTo disable merge by default (#7527)

* UnPackTo disable merge by default

* avoid double free in test

* remove merge parameter

* remove shrink to fit

tests/monster_test.h

@@ -30,6 +30,8 @@ void TypeAliasesTest();
 
 void ParseAndGenerateTextTest(const std::string& tests_data_path, bool binary);
 
+void UnPackTo(const uint8_t *flatbuf);
+
 }  // namespace tests
 }  // namespace flatbuffers