Add tests for adding messages to multiple batches.
Add tests for adding messages to multiple batches.

TouK/vumi,vishwaprakashmishra/xmatrix,vishwaprakashmishra/xmatrix,harrissoerja/vumi,harrissoerja/vumi,TouK/vumi,vishwaprakashmishra/xmatrix,harrissoerja/vumi,TouK/vumi