I am changing one particular  field by checking lastmodifeddate in my code.All works fine but i am struck with test class.I am chnaging multiple records from multiple objects so my batch class looks like below.
 

global class editrecords implements Database.Batchable<string>, Schedulable{
global boolean bool = false;

global Iterable<string> start(Database.BatchableContext ctx) {
    return new list<String> { 'CO__c', 'CO2__c', 'CO3__c','CO4__c'};
}
global void execute(Database.BatchableContext ctx, list<string> listObj) {
    list<sObject> editrecords = new list<sObject>();
    for(string strObjName : listObj) {
        for(sObject objsObject : database.query('Select Id from ' + strObjName + ' where LastModifiedDate < LAST_N_DAYS:36')) {
                editrecords.add(objsObject);
            else {
                b = true;
                break;
            }
        }
    }
    update editrecords;
}
global void finish(Database.BatchableContext ctx) {

}        global void execute(SchedulableContext sc){
         Database.executebatch(new editrecords ());    
    }
}

my test class is as below which get 58% of coverage.How can i get lastmodified date and cover rest of code.please help.
 

@isTest
Public class testeditrecords{

    static testMethod void testMethod1(){

        CO__c CO =new CO__c(Name='testdata');
        insert CO;
        CO2__c co2 =new 
  co2__c(Name='testdata');
        insert co2;
        Co3__c co3 =new Co3__c(Name='test2');
        insert co3;

       for(CO__c c:[Select ID from CO__c where createddate=TODAY]){
        Test.startTest();
       
        editrecords obj = new editrecords();
        Database.executeBatch(obj);
        Test.stopTest();       
        }
   } 
}