Fix lack of warning of unrecognized section names

1. Fix lack of warning by collecting all section names by moving
   m_config_sections.clear() to ArgsManager::ReadConfigFiles().
2. Add info(file name, line number) to warning message.
3. Add a test code to confirm this situation.
3. Do clear() in ReadConfigString().

src/test/util_tests.cpp

@@ -180,9 +180,10 @@ struct TestArgsManager : public ArgsManager
         {
             LOCK(cs_args);
             m_config_args.clear();
+            m_config_sections.clear();
         }
         std::string error;
-        BOOST_REQUIRE(ReadConfigStream(streamConfig, error));
+        BOOST_REQUIRE(ReadConfigStream(streamConfig, "", error));
     }
     void SetNetworkOnlyArg(const std::string arg)
     {