how to cover apexpages.addmessage in test class

Controller: 

public with sharing class mycasecom {
    public String test2 { get; set; }
    public String test1 { get; set; }
    public string test3 {get;set;}
    public string status {get;set;}
    public casecomment ca;
    public case c ;
    public ID ID ;
     public void save(){
        id = ApexPages.currentPage().getParameters().get('caseid');
        system.debug('case id entered' + ID);
        ca = new casecomment();
        ca.ParentId= id;
        ca.commentbody = test1 + '\n' + test2 + '\n' + test3;
        ca.IsPublished = true;
        list<case> c = [select Status from case where ID=:ID];
        list<case> cc = new list<case>();
        for(case cd : c){
            cd.Status = 'Closed';
            cc.add(cd);
        }
        try {
            insert ca;
            update cc;
            status = 'saved';
            system.debug('status value'+ status);
        }catch(DmlException d){
            Apexpages.addMessage(new ApexPages.message(Apexpages.Severity.Error, d.getMessage())) ;
            status = 'unsaved';
        }
    }
}

Test Class

@isTest
public class PopupTestClass {
    @isTest public static void withcaseid(){
        case c = new case(status = 'New',Origin = 'Phone');
        insert c;
        case ca = [select id, status from case where status = 'New'];
        mycasecom cs = new mycasecom();
        cs.Test2=('test1');
        cs.Test1=('test2');
        cs.Test3=('test3');
        apexpages.currentPage().getparameters().put('caseid', ca.id);
        cs.save();
        casecomment cm = [select commentbody, isPublished from casecomment where parentid =: ca.Id];
        string str = cs.test1 +'\n'+ cs.test2 +'\n' +cs.test3;
        system.assertEquals(str , cm.CommentBody);
        system.assert(true, cm.ispublished);
        case g = [select Status from case where ID =: ca.Id];
        system.assertEquals('Closed', g.status);
    }
    @isTest static void withoutcaseid(){
        case c = new case(status = 'New',Origin = 'Phone');
        insert c;
        mycasecom cs = new mycasecom();
        cs.Test2=('test1');
        cs.Test1=('test2');
        cs.Test3=('test3');
        apexpages.currentPage().getparameters().put('caseid', null);
        cs.save();
    }
}