tagschecking: Use gst_message_parse_warning in case of GST_MESSAGE_WARNING
Bus message handler of tags checking unit test uses gst_message_parse_error() in case of GST_MESSAGE_ERROR and GST_MESAGE_WARNING. If gst_message_parse_error() is called in case of GST_MESSAGE_WARNING, assert occurs. So modified to use gst_message_parse_warning() in case of GST_MESSAGE_WARNING.