Why is `$order->getGrandTotal()` returning NULL?

I'm creating a payment module, which has to get the amount of the order in the page that will redirect you to the payment provider.

Here are the lines of code I use in my RedirectBLock:

$session = Mage::getSingleton('checkout/session');
$this->order = Mage::getModel('sales/order')
    ->loadByIncrementId($session->getLastRealOrderId());
$this->amount = sprintf("%.2f", $this->order->getGrandTotal());

When I use the code in a database filled with the sample data from magento, and do the order on my local website, everything is working fine (eg I can see that the grandTotal was retrieved).

However, it doesn't work when I use a test database with Ecomdev_PhpUnit.

Here is my fixture.yml

scope:
  website:
    - website_id: 1
      code: english_website
      name: English Website
      default_group_id: 1
  group:
    - group_id: 1
      website_id: 1
      name: English Store group
      default_store_id: 1
      root_category_id: 1
  store:
    - store_id: 1
      website_id: 1
      group_id: 1
      code: uk
      name: English Store
      is_active: 1
config:
  default/catalog/price/scope: 1
  default/dev/template/allow_symlink: 1
  stores/default/dev/template/allow_symlink: 1
  stores/admin/dev/template/allow_symlink: 1
  stores/french/dev/template/allow_symlink: 1
  stores/german/dev/template/allow_symlink: 1
  websites/admin/dev/template/allow_symlink: 1
  websites/base/dev/template/allow_symlink: 1
  default/carriers/freeshipping/active: 1
eav:
  customer:
    - entity_id: 1
      email: test@mail.com
      password_hash: ''
  catalog_product:
    - entity_id: 1
      type_id: simple
      sku: book
      name: Book
      short_description: Book
      description: Book
      attribute_set_id: 4 # Product entity
      url_key: book
      stock:
        qty: 100.00
        is_in_stock: 1
      website_ids:
        - english_website
      /websites:
        english_website:
          special_price: 7.99
      category_ids:
        - 1 # Default Category
      price: 12.99
      tax_class_id: 2 # Taxable Goods
      status: 1 # Enabled
      visibility: 4 # Visible in Catalog & Search

This is how I trigger the request to the saveOrder route:

$this->reset();
$postData = array(
    "payment"=>array(
        "method"=>"custom"
    ),
);

$this->getRequest()
     ->setMethod('POST')
     ->setPost($postData);

$formKey = Mage::getSingleton('core/session')->getFormKey();

$this->dispatch('checkout/onepage/saveOrder/form_key/'.$formKey);
$this->assertResponseHttpCode(200);
$rawBody = $this->getResponse()
    ->getOutputBody();

$this->assertResponseBodyContains('redirect');