Improve coverage for mutation addSimpleProductToCart or Cart contains a product after product got status Out of Stock

[TomashKhamlai]

Can be related to #437

Preconditions (*)

Steps to reproduce (*)

1. Create Simple Product with
   Name Simple Product
   SKU: simple-product
   Weight: 1
   Price: 1
   Qty: 1000
2. Create empty cart as a Guest or as Customer (GraphQL)
3. Add simple product to Cart (GraphQL)
4. Go to admin
5. Set Simple Product to be Out of Stock
6. Add Simple Product to Cart again (GraphQL) and get this

{
  "errors": [
    {
      "message": "Could not add the product with SKU simple-product to the shopping cart: Product that you are trying to add is not available.",
      "category": "graphql-input",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "addSimpleProductsToCart"
      ]
    }
  ],
  "data": {
    "addSimpleProductsToCart": null
  }
}

7. Try to checkout from GraphQL when [Checkout coverage] Place order concept #404 will be ready

Expected result (*)

1. Checkout is not successful
2. No internal server error

Actual result (*)

1. Cart query

query {
  cart(
    cart_id: "hFWUVh9KPZ4FdgsnpEZDcfLpxW96vm56"
  ) {
    items {
      product {
        sku
      }
      __typename
      qty
      id
    }
  }
}

returns:

{
  "data": {
    "cart": {
      "items": [
        {
          "product": {
            "sku": "simple-product"
          },
          "__typename": "SimpleCartItem",
          "qty": 1,
          "id": "50"
        }
      ]
    }
  }
}

So there could be problems. And tests do not cover such cases