public class BACSPaymentManagerController {
@AuraEnabled
    public static String generateBACSFile(List<Id> selectedPaymentIds, Datetime BACSPaymentDatetime){
        Integer day = BACSPaymentDatetime.day();
        String dayString;
        Integer month = BACSPaymentDatetime.month();
        String monthString;
        Integer year = BACSPaymentDatetime.year();
        String userID = UserInfo.getUserId();
        Date BACSPaymentDate = Date.newInstance(year, month, day);

        if(day > 9)
            dayString = String.Valueof(day);
        else
            dayString = '0'+day;

        if(month > 9)
            monthString = String.Valueof(month);
        else
            monthString = '0'+month;

        String dateLine = 'H,' + year + monthString + dayString + ',' + userID + year + monthString + dayString;
        String terminationString = 'T';
        Integer hour = BACSPaymentDatetime.hour();
        String hourString;
        if(hour > 9)
            hourString = String.Valueof(hour);
        else
            hourString = '0'+hour;

        Integer minute = BACSPaymentDatetime.minute();
        String minuteString;
        if(minute > 9)
            minuteString = String.Valueof(minute);
        else
            minuteString = '0'+minute;
        Integer second = BACSPaymentDatetime.second();
        String secondString;
        if(second > 9)
            secondString = String.Valueof(second);
        else
            secondString = '0'+second;
        String bacsDebitAccountReference = 'SF'+ year + monthString + dayString + hourString + minuteString + secondString;

        Datetime BACSPaymentDatetimeNext4 = BACSPaymentDatetime+4;
        Integer dayNext4 = BACSPaymentDatetimeNext4.day();
        String dayStringNext4;
        Integer monthNext4 = BACSPaymentDatetimeNext4.month();
        String monthStringNext4;
        Integer yearNext4 = BACSPaymentDatetimeNext4.year();

        if(dayNext4 > 9)
            dayStringNext4 = String.Valueof(dayNext4);
        else
            dayStringNext4 = '0'+dayNext4;

        if(monthNext4 > 9)
            monthStringNext4 = String.Valueof(monthNext4);
        else
            monthStringNext4 = '0'+monthNext4;

        bacsDetails__c bacsDetails = bacsDetails__c.getInstance();
        String debitLine = 'D,' + yearNext4 + monthStringNext4 + dayStringNext4 + ',' + bacsDebitAccountReference + ',' + bacsDetails.sortCode__c + '-' + bacsDetails.Account_Number_For_BAS_Debit__c;

        Batch_Reference__c br = new Batch_Reference__c(Name=bacsDebitAccountReference);
        insert br;

        Integer numberOfPayments = 0;
        Decimal totalAmount = 0;

        List<Payment__c> selectedPayment = new List<Payment__c>();
        for(Payment__c pay : [SELECT Id, Name, Amount_Paid__c, Date_Paid__c, Date_Due__c FROM Payment__c WHERE Id IN: selectedPaymentIds]){
            pay.Batch_Reference__c = br.id;
            pay.Date_Paid__c = BACSPaymentDate;
            totalAmount += pay.Amount_Paid__c;
            numberOfPayments += 1;
            selectedPayment.add(pay);
        }
        br.Total_Amount__c = totalAmount;
        br.Number_of_Payments__c = numberOfPayments;
        update selectedPayment;
        update br;

        return '/apex/BACSPaymentFileGenerator?dateLine='+dateLine+'&debitLine='+debitLine+'&id='+br.id;
    }
}

@isTest
public class BACSPaymentManagerControllerTest {
@isTest(SeeAllData=true)
    static void generateBACSFile(){
        Datetime dt = Datetime.newInstance(2018, 5, 1, 8, 0, 0);
        Datetime nextdt = dt+4;
        Account acc = new Account(Name='Aikin Household');
        insert acc;
        system.assert(acc.id!=null);
        Contact con = new Contact(LastName='John Aikin',RSABI_category__c='AB',AccountId=acc.id);
        insert con;
        system.assert(con.id!=null);
        Batch_Reference__c br = new Batch_Reference__c(Name='SF20180501080000');
        insert br;
        Payment__c pay = new Payment__c(Contact__c=con.id,Date_Due__c=system.today(),Payment_Type__c='BACS',Confirm_Payment__c=false,Amount_Paid__c=100,Batch_Reference__c=br.id);
        insert pay;
        system.assert(pay.id!=null);
        List<Id> selectedPaymentId = new List<Id>();
        selectedPaymentId.add(pay.id);


        Profile p = [SELECT Id FROM Profile WHERE Name='Standard User'];
        User u2 = new User(Alias = 'newUser', Email='newuser@testorg.com',
                           EmailEncodingKey='UTF-8', LastName='Testing', LanguageLocaleKey='en_US',
                           LocaleSidKey='en_US', ProfileId = p.Id,
                           TimeZoneSidKey='America/Los_Angeles', UserName='newuser@testorg.com');
        insert u2;
        bacsDetails__c bacsDetails = bacsDetails__c.getInstance();

        String dateLine = 'H,20180501,'+u2.id+'20180501';
        String debitLine='D,20180505,SF20180501000000,'+bacsDetails.sortCode__c + '-' + bacsDetails.Account_Number_For_BAS_Debit__c;

        test.startTest();
        system.runAs(u2){
            String returnString =  BACSPaymentManagerController.generateBACSFile(selectedPaymentId,dt);
            system.assertEquals('/apex/BACSPaymentFileGenerator?dateLine='+dateLine+'&debitLine='+debitLine+'&id='+br.id, returnString );

        }
        test.stopTest();
    }
}

System.AssertException: Assertion Failed: Expected: /apex/BACSPaymentFileGenerator?dateLine=H,20180501,0056E0000036LiVQAU20180501&debitLine=D,20180505,SF20180501000000,804121-00792403&id=a1X6E000000c8NqUAI, Actual: /apex/BACSPaymentFileGenerator?dateLine=H,20180501,0056E0000036LiVQAU20180501&debitLine=D,20180505,SF20180501000000,804121-00792403&id=a1X6E000000c8NrUAI

Batch_Reference__c br = new Batch_Reference__c(Name=bacsDebitAccountReference);
insert br;

 for(Payment__c pay : [SELECT Id, Name, Amount_Paid__c, Date_Paid__c, Date_Due__c FROM Payment__c WHERE Id IN: selectedPaymentIds]){
        pay.Batch_Reference__c = br.id;
        pay.Date_Paid__c = BACSPaymentDate;
        totalAmount += pay.Amount_Paid__c;
        numberOfPayments += 1;
        selectedPayment.add(pay);
    }