I 'm writing this class with its test class and I can save my code because there aren't errors but test class don't cover code of the class.
Can we help me to solve problems? 
Thank you.

This is Class : 

public class Ordine_Aziendale {


   public Quote preventivo{get; set;}
   public List<Account> account{get; set;}
   public Opportunity opportunita{get; set;}
   public List<QuoteLineItem> qtl{get; set;}
   public Integer cont{get;set;}
   
   public User utente{get; set;}  
    
    public Ordine_Aziendale(ApexPages.StandardController stdcontroller) {
    
      this.preventivo = (Quote)stdController.getRecord();
    
   // Product2 p2 = new Product2 (name='test');
      //  insert p2;
        
        qtl = [SELECT id , product2.id, product2.name, product2.ProductCode, Quantity, UnitPrice FROM QuoteLineItem  WHERE Quote.id =: preventivo.id];
        cont = qtl.size();
        
      // account = [SELECT id,Name,BillingAddress,ShippingAddress FROM Account WHERE id =: preventivo.id];
     // opportunita = [SELECT id,Name,BillingAddress,ShippingAddress FROM Account WHERE id =: preventivo.AccountId];
    }
}


This is my test class :

@isTest
public class Ordine_Aziendale_Test {
    static testMethod void Costruttore_Test()
    {
     
 /************************* START TEST   *************************************/      
 
     test.startTest();
      
     List<Account> l_acc = new List<Account>();
     Account acc = new Account(name='Test Account', billingStreet='Test', billingCity='Test', billingState='Test', billingCountry='Test');
     insert acc;
     l_acc.add(acc);
  
     List<Opportunity> l_opp = new List<Opportunity>();
     Opportunity opp = new Opportunity();
     opp.Accountid = acc.id;
     opp.Name = 'test';
     opp.StageName = 'Prospecting';
     opp.CloseDate = date.today();
     opp.Type = 'New Client';
     opp.NextStep = 'Test';
     opp.LeadSource = 'Business Development';
     insert opp;
     l_opp.add(opp);
      
     List<Quote> l_preventivi = new List<Quote>();  
     Quote preventivo = new Quote(Name = 'Test',OpportunityId = opp.id, N_Offerta__c = 'test');
     insert preventivo;
     l_preventivi.add(preventivo);
     
     // Pricebook2 standardPB = [select id from Pricebook2 where isStandard=true];
        
     Pricebook2 pb = new Pricebook2(Name = 'Standard Price Book 2009', Description = 'Price Book 2009 Products', IsActive = true);
     insert pb;
        
      Product2 prod = new Product2(Name = 'Anti-infectives 2007', Family = 'Best Practices', IsActive = true);
      insert prod;

     List<QuoteLineItem> l_qtl = new List<QuoteLineItem>();
     QuoteLineItem qtl = new quotelineitem(quoteid = preventivo.id,quantity = 1,unitprice = 1); 
     insert qtl;
     l_qtl.add(qtl);

      ApexPages.StandardController controller = new ApexPages.StandardController(preventivo);
      Ordine_Aziendale stdController = new Ordine_Aziendale(controller);



/********************************************** FINISH TEST   *************************************/      
 
   test.stopTest();

   }
  
 }