Wondering how to test a drupal_http_request()

I'm writing tests for xmlsitemap_engines:

This is the code that runs in xmlsitemap_engines_cron():

  foreach ($engines as $engine) {
    $url = xmlsitemap_engines_prepare_url(xmlsitemap_engines_get_engines($engine, 'url'));
    $request = drupal_http_request($url);
    watchdog('xmlsitemap', 'Submitted the sitemap to %url.', array('%url' => $url));
  }

And I have an xmlsitemap_engines_test.module that implements the following:

/**
 * Implementation of hook_menu().
 */
function xmlsitemap_engines_test_menu() {
  $items['ping'] = array(
    'page callback' => 'xmlsitemap_engines_test_pinged',
    'access callback' => TRUE,
    'type' => MENU_CALLBACK,
  );
  return $items;
}

/**
 * Implementation of hook_xmlsitemap_engines_alter().
 */
function xmlsitemap_engines_test_xmlsitemap_engines_alter(&$engines) {
  $engines['simpletest'] = array(
    'name' => t('SimpleTest'),
    'url' => url('ping', array('absolute' => TRUE, 'query' => 'sitemap=[sitemap]')),
  );
}

function xmlsitemap_engines_test_pinged() {
  if (empty($_GET['sitemap'])) {
    watchdog('xmlsitemap', 'No sitemap parameter provided.', NULL, WATCHDOG_WARNING);
  }
  else {
    watchdog('xmlsitemap', 'Recieved ping for @sitemap.', array('@sitemap' => $_GET['sitemap']));
  }
}

And this is the code in xmlsitemap_engines.test:

  function testPing() {
    $edit = array('xmlsitemap_engines_engines[simpletest]' => TRUE);
    $this->drupalPost('admin/settings/xmlsitemap/engines', $edit, t('Save configuration'));
    $this->assertText(t('The configuration options have been saved.'));

    drupal_cron_run();
    $query = db_query("SELECT type, message, variables, severity, link FROM {watchdog} ORDER BY wid DESC");
    while ($watchdog = db_fetch_array($query)) {
      $this->assertTrue(TRUE, var_export($watchdog, TRUE));
    }
  }

The test results return the watchdog/dblog records that were called in xmlsitemap_engines_cron() but none of the watchdog records in xmlsitemap_engines_test_pinged(). I do verify that the code is being executed correctly because I added some log-file-writing calls in xmlsitemap_engines_test_pinged().

So the main problem is this: drupal_http_request() causes the $_SERVER['HTTP_USER_AGENT'] to be changed to Drupal (+http://drupal.org/). Now when the xmlsitemap_engines_test_pinged() code executes, the SimpleTest useragent is not found, and so the normal database is used, instead of the 'testing' database. I verified this by adding this line to settings.php and the result of the log file:

+ file_put_contents('./log.txt', request_uri() . ' ' . $_SERVER['HTTP_USER_AGENT'] . "\n", FILE_APPEND);
/admin/build/testing Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.10) Gecko/2009042523 Linux Mint/6 (Felicia) Firefox/3.0.10
/batch?op=start&id=342 Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.10) Gecko/2009042523 Linux Mint/6 (Felicia) Firefox/3.0.10
/batch?id=342&op=do Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.10) Gecko/2009042523 Linux Mint/6 (Felicia) Firefox/3.0.10
/user simpletest881040
/user simpletest881040
/user/3 simpletest881040
/admin/settings/xmlsitemap/engines simpletest881040
/admin/settings/xmlsitemap/engines simpletest881040
/admin/settings/xmlsitemap/engines simpletest881040
/ping?sitemap=http://mysql.drupal6dev.local/sitemap.xml Drupal (+http://drupal.org/)
/ simpletest881040
/batch?id=342&op=finished Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.10) Gecko/2009042523 Linux Mint/6 (Felicia) Firefox/3.0.10
/admin/build/testing Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.10) Gecko/2009042523 Linux Mint/6 (Felicia) Firefox/3.0.10

Any ideas on how best to run these tests when I want to test that the drupal_http_request() is performed correctly? Should I just not try to test that the request was received and just test that it was sent properly? Help me boombatower and community! :)