Add basic test for supplies

Before changing this part of the code it is good to know that we are not
going to break anything.

In order to make testing easier I decided to add `id` of product to each
table row

rails_application/app/views/products/index.html.erb

@@ -20,7 +20,7 @@
 
   <tbody>
   <% @products.each do |product| %>
-    <tr class="border-t">
+    <tr class="border-t" id=<%= dom_id(product) %>>
       <td class="py-2"><%= link_to product.name, product_path(product), class: "hover:underline text-blue-500" %></td>
       <td class="py-2"><%= number_to_currency product.price %></td>
       <td class="py-2"><%= product.vat_rate %></td>

rails_application/test/integration/supplies_test.rb

@@ -0,0 +1,50 @@
+require "test_helper"
+
+class SuppliesTest < InMemoryRESIntegrationTestCase
+  def setup
+    super
+  end
+
+  def test_supply_new_product
+    create_product
+    product = Product.find_by(sku:)
+    increase_stock_level_by_10(product.id)
+
+    follow_redirect!
+
+    assert_select "tr#product_#{product.id}" do
+      assert_select "td", "10"
+    end
+    assert_equal(10, product.reload.stock_level)
+  end
+
+  def test_supply_product_with_existing_stock
+    create_product
+    product = Product.find_by(sku:)
+    increase_stock_level_by_10(product.id)
+
+    assert_changes -> { Product.find(product.id).stock_level }, from: 10, to: 20 do
+      increase_stock_level_by_10(product.id)
+    end
+
+    follow_redirect!
+
+    assert_select "tr#product_#{product.id}" do
+      assert_select "td", "20"
+    end
+  end
+
+  private
+
+  def increase_stock_level_by_10(product_id)
+    post "/products/#{product_id}/supplies", params: { product_id: product_id, quantity: 10 }
+  end
+
+  def create_product
+    post "/products", params: { product: { name: "Stanley Cup", price: 100, vat_rate: 23, sku: } }
+  end
+
+  def sku
+    "SKU-ST4NL3Y"
+  end
+end