Unit testing DefaultHttpRequestRetryHandler

I'm working on some legacy code that stores files to a remote server. I'd like to use Apache's DefaultHttpRequestRetryHandler to implement a retry logic. A simplified version of the implementation is shown below. How do I test my retry logic?

I was able to manually test it by overriding retryRequest() in the DefaultHttpRequestRetryHandler class but an automated way would be nice. (I'm using Spock to test.)

   private CloseableHttpClient getHttpClient() {
        DefaultHttpRequestRetryHandler retryHandler = new DefaultHttpRequestRetryHandler();
        CloseableHttpClient httpClient = HttpClients.custom().setRetryHandler(retryHandler).build();
        return httpClient;
   }

   public CloseableHttpResponse uploadFile(){    
        CloseableHttpClient httpClient = getHttpClient();
        CloseableHttpResponse response = null;
        try {
            response = httpClient.execute(post, getHttpContext());
        } catch (Exception ex) {
            //handle exception
        }
        return response;    
   }