In #14708, It was introduced that to warn when unrecognized section names are exist in the config file.
But ```m_config_sections.clear()```  in ```ArgsManager::ReadConfigStream()``` is called every time when reading each configuration file, so it can warn about only last reading file if ```includeconf``` exists.

This PR fix lack of warning by collecting all section names by moving ```m_config_sections.clear()```  to ```ArgsManager::ReadConfigFiles()``` .
Also add a test code to confirm this situation.

ninja check
test_runner.py feature_config_args.py