[TestMessageHandler] print better error message and reports the backlog in the gtest failure.
[TestMessageHandler] print better error message and reports the backlog in the gtest failure.

Anatoscope/sofa,Anatoscope/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa,hdeling/sofa,FabienPean/sofa,FabienPean/sofa,hdeling/sofa,hdeling/sofa,Anatoscope/sofa,FabienPean/sofa,Anatoscope/sofa,FabienPean/sofa,FabienPean/sofa,Anatoscope/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,hdeling/sofa,Anatoscope/sofa,hdeling/sofa,FabienPean/sofa