My Apex class:
public class OpenEventsforPotentials {

public List<OpenActivity> open{get;set;}

    public Boolean refreshPage {get; set;}

    public List<Event> accs { get; set;}

    public Id Potentialld{get;set;}
    public Id Accountid{get;set;}

    public string SelectedEventId { get; set;}

    public OpenEventsforPotentials (ApexPages.StandardController controller) {

        Potentialld= ApexPages.CurrentPage().getparameters().get('id');
        list<Account> Accountid=[select id from Account]; 
        list<Opportunity>   c =[SELECT Name,

                                (Select Id,Subject,IsTask, WhoId, ActivityDate,Status, Priority, OwnerId FROM OpenActivities WHERE IsTask=False)

                           FROM Opportunity WHERE Id = :Potentialld];

        if(c.size() > 0){

            open = c[0].OpenActivities;

        }

     

    }

       

   

    public pageReference DeleteEvent(){

     

        // if for any reason we are missing the reference

        if (SelectedEventId != null) {

            Event tobeDeleted = [SELECT Id FROM Event WHERE Id =:SelectedEventId];

            if(tobeDeleted != null){

                delete tobeDeleted;

            }

             

            for(Integer i=0;i < open.size();i++){

                if(open[i].Id == SelectedEventId){

                    open.remove(i);

                    i--;

                }

            }

        }

         

        return null;

    }

    public Pagereference refresh(){

        PageReference secondPage = new PageReference('https://ap1.salesforce.com/{!Leadld}');

       secondPage.getParameters().put('id',Potentialld);

       secondPage.setRedirect(true);

         

        return secondPage;

     

    }   

}
Test class:
@isTest
public class  Testclassforcases{

    static TestMethod void testforcases(){

       User usr = [SELECT Id From User WHERE Id =:UserInfo.getUserID()];

        System.runAs(usr){
           Account A1=[select id from account
                       where id = : case__c.Account_Name__c];
           case__c case1=new case__c(OwnerId=usr.Id,CurrencyIsoCode = 'INR',Name=A1.Id
                                     );

            Insert case1;
           

             

           
            
            Event event1=new Event(OwnerId=usr.Id,Subject='cc',DurationInMinutes=60,ActivityDateTime=Datetime.newInstance(2015,25,2,00,00,00),
                                 EndDateTime=Datetime.newInstance(2015,25,2,01,00,00),StartDateTime=DateTime.newInstance(2015,25,2,00,00,00));
            
            Insert event1;
        

            ApexPages.StandardController stdcontroller = new ApexPages.Standardcontroller(case1);

            ApexPages.CurrentPage().getparameters().put('id',case1.Id);
            OpenEventsforCases eve=new OpenEventsforCases(stdcontroller);
            eve.DeleteEvent();
           // eve.refresh();
            OpenTasksforCases eve1= new OpenTasksforCases( stdcontroller);
            eve1.listwrapper[0].checked = true;
            eve1.close();

         

        }

    } 
}