Fix add image assets when creating product fixture

Reorder the product fixture `create()` method to prevent the `save()` method being called prematurely on the product model when adding images to the media gallery.

I discovered this issue whilst trying to set an image when creating a product fixture. This gave me the following error when running Behat:

```
Unable to create the Magento fixture SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (`magentodb`.`catalog_product_entity`, CONSTRAINT `FK_CAT_PRD_ENTT_ATTR_SET_ID_EAV_ATTR_SET_ATTR_SET_ID` FOREIGN KEY (`attribute_set_id`) REFERENCES `eav_attribute_set` (`attribute_set_id`) ON )
```

Eventually I found this was related to the attribute set not existing, more specifically, no attribute set id was used when the `save()` method was first called on the product. This was due to the `setProductImageAssets()` saving the product when adding images to the media gallery, prior to the attribute data being set on the model.

Deferring this process, and removing the duplicate call to `save()`, allowed me to successfully create product fixtures both with and without images.