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.
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