There is no control type as sap.ui.commons.MessageBox. You'll have to find the css classes that MessageBox uses and try to fetch the instance using the class selector. Below is a sample code that I used as matcher for sap.m.MessageBox success.
fnShouldSeeSuccessMessage : function(sViewName,sSuccessMessage,sErrorMessage){ return this.waitFor({ pollingInterval : 100, viewName : sViewName, check : function(){ return !!sap.ui.test.Opa5.getJQuery()(".sapMMessageBoxSuccess").length && sap.ui.test.Opa5.getJQuery()(".sapMMessageBoxSuccess").find(".sapMText").text() === sSuccessMessage;
}, success : function(oMessagebox){ ok (true, sSuccessMessage); }, errorMessage : sErrorMessage });
},