Add a class to an element after AJAX
Taking a blind guess: Your context matches the entire parent container, so you can't filter by "new ones only" with context. But as a workaround you could specify a more explicit find to filter only new ones:

This will add .i-was-here-first to every views row present on document ready:

 $(document).ready(function($) {
    $('.views-infinite-scroll-content-wrapper').find('.views-row').addClass('i-was-here-first');
});


Now use that class to filter sub-sequent behavior calls by using a more explicit find():

Drupal.behaviors.testing = {
    attach:function (context, settings) {
      $('.views-infinite-scroll-content-wrapper', context).find('.views-row:not(.i-was-here-first)').addClass('i-was-here-second');
    }
};


Side note:
I'd recommended not to use $(document).ready() at all in Drupal because this might cause problems when using BigPipe module, which is active by default. Drupal will initially trigger all behaviors with document as context, so you should use something like this instead:

Drupal.behaviors.instead_of_document_ready_do_this = {
  attach: function (context, settings) {
    $(document, context).once('my_unique_name').each( function... );
  }
}