Test that we refresh the cache on variant destroy

These tests prove that
https://github.com/openfoodfoundation/openfoodnetwork/issues/3629 is
indeed is a bug because we don't refresh the cache when deleting
a variant.

spec/controllers/spree/admin/variants_controller_spec.rb

@@ -59,6 +59,19 @@ module Spree
             spree_delete :destroy, id: variant.id, product_id: variant.product.permalink, format: 'js'
             expect(response).to render_template('spree/admin/shared/_destroy')
           end
+
+          it 'refreshes the cache' do
+            expect(OpenFoodNetwork::ProductsCache).to receive(:variant_destroyed).with(variant)
+            spree_delete :destroy, id: variant.id, product_id: variant.product.permalink, format: 'js'
+          end
+
+          it 'destroys all its exchanges' do
+            exchange = create(:exchange)
+            variant.exchanges << exchange
+
+            spree_delete :destroy, id: variant.id, product_id: variant.product.permalink, format: 'js'
+            expect(variant.exchanges).to be_empty
+          end
         end
 
         context 'when requesting with html' do
@@ -85,6 +98,19 @@ module Spree
               product_id: variant.product.permalink
             )
           end
+
+          it 'refreshes the cache' do
+            expect(OpenFoodNetwork::ProductsCache).to receive(:variant_destroyed).with(variant)
+            spree_delete :destroy, id: variant.id, product_id: variant.product.permalink, format: 'js'
+          end
+
+          it 'destroys all its exchanges' do
+            exchange = create(:exchange)
+            variant.exchanges << exchange
+
+            spree_delete :destroy, id: variant.id, product_id: variant.product.permalink, format: 'js'
+            expect(variant.exchanges).to be_empty
+          end
         end
       end
     end