Add media gallery to product fixture with updated product spec

Allow products to be created with an image. 

```text
    Given the following products exist:
      | name | sku  | type_id | price | tax_class_id | image                                          | category_ids |
      | test    | test  | simple  | 10.00 | 2                  | features/assets/placeholder.png | 2                   |
```