public class AccountRelatedContacts  {
        public id tobeEdited{get; set;}
        public String selectedcouId{get;set;}
        public String selectedAccId{get;set;}
        public string conid{get;set;}
        public boolean showPopup{ get; set; }
         public boolean isDownload {get;set;}

        public list<contact> lstcontact{get;set;}


         public List<SelectOption> getCountryNames() {

                 List<SelectOption> couOptions= new List<SelectOption>();
                          couOptions.add( new SelectOption('','--Select--'));
                          list<Country__c> cut = [select Id,name from Country__c];
                          for( Country__c  cou : cut ) {
                                  couOptions.add( new SelectOption(cou.Id,cou.name));
                          }
                         return couOptions;
                   }

               public List<SelectOption> getAccountNames() {
               system.debug('country ID'+selectedcouid);
                  List<SelectOption> accOptions= new List<SelectOption>();
                  List<SelectOption> options = new List<SelectOption>();
                  if(selectedcouId != null){

                  list<account> act = [select Id,name,Country__c from account where Country__c=:selectedcouId order by name ASC ];

                  for( Account acc : act  ) {
                          accOptions.add( new SelectOption(acc.Id,acc.name));
                  }
                  }
               else
                    {
                        accOptions.add( new SelectOption('--None--','--None--'));
                    }
             return accOptions;
       }

      public pagereference Submit(){

      lstcontact= new List<contact>();
      lstcontact= [SELECT ID,name, Firstname,lastname,email,phone,account.name from contact WHERE accountId=:selectedaccid order by name ASC];
      return null;
    }
}

public class testCountryAccountRelatedContacts  {

private static testmethod void testgetCountryNames() {
country__C c = new country__c();
insert c;
CountryAccountRelatedContacts controller = new CountryAccountRelatedContacts();
controller.selectedcouId = c . id;
   List<SelectOption> sp = controller.getCountryNames();
   system.assertEquals(sp.size(),2);
    }
     private static testMethod void testgetaccountNames()
 {
country__C c = new country__c();
insert c;
account acc = new account(Country__c=c.id,name = 'test');
insert acc;
CountryAccountRelatedContacts controllers = new CountryAccountRelatedContacts();
List<SelectOption> sp1 = controllers.getaccountNames();
     controllers.selectedcouId = c.id;
    controllers.selectedAccId = acc.id;
    string getselectedcouId =  controllers.selectedcouId;
    string getsselectedAccId =  controllers.selectedAccId;
    List<SelectOption> sp2 = controllers.getaccountNames();
     system.assertEquals(sp2.size(),2);
}
}

USER_DEBUG [33]|DEBUG|country IDnull