When a Visualforce Page1 calls Visualforce Page2 and Visualforce Page2 has a extension to an Apex class with a constructor that contains the following line of code

"ApexPages.currentPage().getHeaders().get('Referer')"

How can the class be tested when being called by a Apex test class instead of a Visualforce page?

The result of "ApexPages.currentPage().getHeaders().get('Referer')" is setting a class property that is being used by other methods of the class.

//*****************************************************

public class Payment_Extension
{
    private Payment__c payment;
    private ApexPages.StandardSetController myController;
    private Payment currentPayment;
    private boolean isReceivable;    
    
    //********************************************************************************
    public Payment_Extension(ApexPages.StandardSetController stdController)
    {
        system.debug('PaymentReceivable_Extension');
        payment = (Payment__c)stdController.getRecord();
        myController = stdController;
        isReceivable = ApexPages.currentPage().getHeaders().get('Referer').contains('tabspayreceivable');     
    }
    
    //********************************************************************************
    public List<selectOption> getReceivableTypes()
    {
        if(isReceivable)
        {
            return currentPayment.ReceivableTypes();
        }
        else
        {
            return currentPayment.PayableTypes();
        }
    }
}

//*****************************************************