OpportunityLineItem.Total_w_Tax__c -this is formula field 

getting above error in test class

please see the test class below

@isTest
private class ItemizedReceiptTest {
static testMethod void testMethodPostive(){
    Product2 prod = new Product2(
        Name = 'Test',
        IsActive = true
    );
    insert prod;
    Account acc = new Account(Name ='Test');
    insert acc;
    Opportunity testOpp = new Opportunity();
    testOpp.Name='Test Opportunity' ;
    testOpp.AccountId=acc.id;
    testOpp.StageName='Order Placed';
    testOpp.CloseDate= Date.newInstance(2019,03,28);
    insert testOpp;
    Id pricebookId = Test.getStandardPricebookId();
    //Create your pricebook entry
    PricebookEntry pbEntry = new PricebookEntry(
        Pricebook2Id = pricebookId,
        Product2Id = prod.Id,
        UnitPrice = 100.00,
        IsActive = true
    );
    insert pbEntry;
    //create your opportunity line item.  This assumes you already have an opportunity created, called opp
    OpportunityLineItem oli = new OpportunityLineItem(
        OpportunityId = testOpp.Id,
        Quantity = 5,
        PricebookEntryId = pbEntry.Id,
        TotalPrice = pbEntry.UnitPrice,
        GST__c = 7,
        PST__c = 7,
        Total_w_Tax__c = 8 // getting error here
    );
    insert oli;
    ApexPages.StandardController stdSetController = new ApexPages.StandardController(testOpp);
    ItemizedReceipt obj = new ItemizedReceipt(stdSetController);
    obj.total();
} 
}



class

public with sharing class ItemizedReceipt {
   
    public List<OpportunityLineItem> oppLst          { set; get; } //List of Products in a Opportunity
    public Double                    totalPrice_cal  { set; get; } //Variable to calculate Total Price of all the products
    public Double                    totalGST_cal    { set; get; } //Variable to calculate GST of all the products
    public Double                    totalPST_cal    { set; get; } //Variable to calculate PST of all the products
    public Double                    totalAmt_cal    { set; get; } //Variable to calculate Total Amount of all the products
    public DateTime                  getCloseDate    { set; get; } //Variable to get the Created Date
    public String                    dayOfWeek       { set; get; } //Variable to store the Day from the Created Date
    public String                    accName         { set; get; } //Variable to store the Customer Name
   
    public  ItemizedReceipt(ApexPages.StandardController controller) {
        if ( controller.getId() != null ) {
            oppLst = [SELECT Product2.Name, Opportunity.Id, Opportunity.CloseDate, TotalPrice, GST__c, PST__c,
                      Total_w_Tax__c, Opportunity.Account.Name
                      FROM   OpportunityLineItem
                      WHERE  Opportunity.Id=:controller.getId()
                     ];
            getCloseDate = oppLst[0].Opportunity.CloseDate;
            dayOfWeek = getCloseDate.format('EEEE');
            accName = (string)oppLst[0].Opportunity.Account.Name;
            totalPrice_cal = 0.0;
            totalGST_cal = 0.0;
            totalPST_cal = 0.0;
            totalAmt_cal = 0.0;
            total();
        }
    }  
   
    //Method calculates all the totals
    public  void total() {
        for ( integer i = 0; i < oppLst.size(); i++ ) {
            totalPrice_cal += oppLst[i].TotalPrice;
            totalGST_cal   += oppLst[i].GST__c;
            totalPST_cal   += oppLst[i].PST__c;
            totalAmt_cal   += oppLst[i].Total_w_Tax__c;
        }
    }
}