I have this code to auto-create OLI when create a new opportunity and selet a product name from the custome picklist (product__C).

I don't know how to write test code... plz help me..

trigger AutoCreateProduct on Opportunity (after insert) { 
    List<OpportunityLineItem> OpportunityLineItems = new List<OpportunityLineItem>(); 
    for (Opportunity newOpportunity: Trigger.New) { 
        if(newOpportunity.product__C.equalsIgnoreCase('CRA')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001a8iVAAQ',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        } if (newOpportunity.product__C.equalsIgnoreCase('GALV')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001a8iQAAQ',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
          if (newOpportunity.product__C.equalsIgnoreCase('2TFS')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBo7AAE',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('DRTP')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBoRAAU',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('SECONDARY TIN')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBoqAAE',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('DRBP')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBoMAAU',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('GLHR')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBobAAE',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('1TFS')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBo2AAE',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('ETP')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBoWAAU',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('SECONDARY SHEET')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBolAAE',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('SRTP')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBp0AAE',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('CRFH')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBoCAAU',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('CRHS')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBoHAAU',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('HRP')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBogAAE',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('STS')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBp5AAE',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }
if (newOpportunity.product__C.equalsIgnoreCase('SRBP')){
            OpportunityLineItems.add(new OpportunityLineItem(OpportunityId = newOpportunity.Id,PricebookEntryId ='01u61000001aBovAAE',Quantity = newopportunity.volume__C,UnitPrice = newopportunity.Sales_Price__C)); 
        }

        
        
        
    } 
    if(!OpportunityLineItems.isEmpty()){
        Database.SaveResult[] srList = Database.insert(OpportunityLineItems, false);

        // Iterate through each returned result
        for (Database.SaveResult sr : srList) {
            if (sr.isSuccess()) {
                // Operation was successful, so get the ID of the record that was processed
                System.debug('Successfully inserted Line Item. Oppty Prodcut ID: ' + sr.getId());
            }
            else {
                // Operation failed, so get all errors                
                for(Database.Error err : sr.getErrors()) {
                    System.debug('The following error has occurred.');                    
                    System.debug(err.getStatusCode() + ': ' + err.getMessage());
                    System.debug('Opportunity Product fields that affected this error: ' + err.getFields());
                }
            }
        }
    }






@IsTest (seealldata=true)
public class AutoCreateProduct{
    public static testmethod void myunittest(){
               
       //create a dummy account
        Account a = new Account ();
        a.Name = 'Test Asset Account';
        insert a;
        
        //create a dummy opportunity
        Opportunity o = new Opportunity();
        o.AccountId = a.Id;
        o.Name = 'test Oppty';
        o.StageName = 'opportunity';
        o.CloseDate = system.today();
        o.product__c = 'cra';
        o.volume__c = 10;
                insert o;
  
    }
}