Add test for VumiMessage field validator.
Add test for VumiMessage field validator.

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