I've defined some custom bypassing function as below:

/**
 * Implementation of hook_menu().
 */
function foo_menu() {

  // URL for user to be directed to on test order completion
  $items['test/order/%/complete'] = array(
    'title' => 'Fake payment bypass.',
    'page callback' => 'foo_test_order_complete',
    'page arguments' => array(2),
    // 'access callback' => variable_get('my_environment', 'live') <> 'live', // disabled on live
    'access callback' => 'user_access',
    'access arguments' => array('administer site configuration'), // Only admin can test that
    'type' => MENU_CALLBACK,
  );

  return $items;
}

/**
 * Test order completion page
 */
function foo_test_order_complete($order_id) {

  if (empty($_SESSION['cart_order'])) {
    $_SESSION['cart_order'] = $order_id;
  }

  $order = uc_order_load($_SESSION['cart_order']);

  // This lets us know it's a legitimate access of the complete page.
  $_SESSION['do_complete'] = TRUE;
  module_load_include('inc', 'uc_cart', 'uc_cart.pages');
  uc_cart_checkout_complete();
  drupal_goto('cart/checkout/complete');

}

So when submit your Review order, after that you can go to:

test/order/12345/complete where 12345 is number of your order or just:

test/order/current/complete to load the order from the session.

These URLs are accessible only by admin, but you can free to extend the permissions as you wish.

Drupal 6

I've defined some custom bypassing function as below:

/**
 * Implementation of hook_menu().
 */
function foo_menu() {

  // URL for user to be directed to on test order completion
  $items['test/order/%/complete'] = array(
    'title' => 'Fake payment bypass.',
    'page callback' => 'foo_test_order_complete',
    'page arguments' => array(2),
    // 'access callback' => variable_get('my_environment', 'live') <> 'live', // disabled on live
    'access callback' => 'user_access',
    'access arguments' => array('administer site configuration'), // Only admin can test that
    'type' => MENU_CALLBACK,
  );

  return $items;
}

/**
 * Test order completion page
 */
function foo_test_order_complete($order_id) {

  if (empty($_SESSION['cart_order'])) {
    $_SESSION['cart_order'] = $order_id;
  }

  $order = uc_order_load($_SESSION['cart_order']);

  // This lets us know it's a legitimate access of the complete page.
  $_SESSION['do_complete'] = TRUE;
  module_load_include('inc', 'uc_cart', 'uc_cart.pages');
  uc_cart_checkout_complete();
  drupal_goto('cart/checkout/complete');

}

So when submit your Review order, after that you can go to:

test/order/12345/complete where 12345 is number of your order or just:

test/order/current/complete to load the order from the session.

These URLs are accessible only by admin, but you can free to extend the permissions as you wish.

I've defined some custom bypassing function as below:

/**
 * Implementation of hook_menu().
 */
function foo_menu() {

  // URL for user to be directed to on test order completion
  $items['test/order/%/complete'] = array(
    'title' => 'Fake payment bypass.',
    'page callback' => 'foo_test_order_complete',
    'page arguments' => array(2),
    // 'access callback' => variable_get('my_environment', 'live') <> 'live', // disabled on live
    'access callback' => 'user_access',
    'access arguments' => array('administer site configuration'), // Only admin can test that
    'type' => MENU_CALLBACK,
  );

  return $items;
}

/**
 * Test order completion page
 */
function foo_test_order_complete($order_id) {

  if (empty($_SESSION['cart_order'])) {
    $_SESSION['cart_order'] = $order_id;
  }

  $order = uc_order_load($_SESSION['cart_order']);

  // This lets us know it's a legitimate access of the complete page.
  $_SESSION['do_complete'] = TRUE;
  uc_cart_checkout_complete();
}

So when submit your Review order, after that you can go to:

test/order/12345/complete where 12345 is number of your order or just:

test/order/current/complete to load the order from the session.

These URLs are accessible only by admin, but you can free to extend the permissions as you wish.

I've defined some custom bypassing function as below:

/**
 * Implementation of hook_menu().
 */
function foo_menu() {

  // URL for user to be directed to on test order completion
  $items['test/order/%/complete'] = array(
    'title' => 'Fake payment bypass.',
    'page callback' => 'foo_test_order_complete',
    'page arguments' => array(2),
    // 'access callback' => variable_get('my_environment', 'live') <> 'live', // disabled on live
    'access callback' => 'user_access',
    'access arguments' => array('administer site configuration'), // Only admin can test that
    'type' => MENU_CALLBACK,
  );

  return $items;
}

/**
 * Test order completion page
 */
function foo_test_order_complete($order_id) {

  if (empty($_SESSION['cart_order'])) {
    $_SESSION['cart_order'] = $order_id;
  }

  $order = uc_order_load($_SESSION['cart_order']);

  // This lets us know it's a legitimate access of the complete page.
  $_SESSION['do_complete'] = TRUE;
  module_load_include('inc', 'uc_cart', 'uc_cart.pages');
  uc_cart_checkout_complete();
  drupal_goto('cart/checkout/complete'); 

}

So when submit your Review order, after that you can go to:

test/order/12345/complete where 12345 is number of your order or just:

test/order/current/complete to load the order from the session.

These URLs are accessible only by admin, but you can free to extend the permissions as you wish.