Writing test classes for external object in salesforce

Main class example

public without sharing class MyOrderHistoryController {

    @TestVisible private static list<SoldToSet__x> mockedcustlist = new List<SoldToSet__x>();

    @AuraEnabled

    //filter values for the report based on logged in customer

    public static list<SoldSet__x> getCustomers( string soldtono){

        list<SoldSet__x> custlist=new list<SoldSet__x>();

        //query Sold To account details

        if(!Test.isRunningTest()){

                for(SoldSet__x oh: [SELECT Flag__c,City__c,Name__c,SoldToNr__c FROM SoldSet__x                                                  WHERE SoldToNr__c IN:soldtono]){

                                custlist.add(oh);

                }

       }else{

                custlist.addAll(mockedcustlist);

        }

        return custlist;

    }

Test class  

@isTest

public without sharing class MyOrderHistoryControllertest {

     static testMethod void myTest() {     

                SoldSet__x mockedCust1 = new SoldSet__x(

                  SoldToNr__c='600459',

                  Name__c='abcLtd',

                  City__c='test1',

                  Flag__c='YES'

                );

                SoldSet__x mockedCust2 = new SoldSet__x(

                  SoldToNr__c='600459',

                  Name__c='xyzLtd',

                  City__c='test2',

                  Flag__c='NO'

                );

                MyOrderHistoryController.mockedcustlist.add(mockedCust1);

                MyOrderHistoryController.mockedcustlist.add(mockedCust2);

                //run as portal user

                system.runas(u){

                         list<SoldSet__x> accs=new list<SoldSet__x>();

                         accs = MyOrderHistoryController.getCustomers();

                         System.assert(accs != null);

                }

        }

}