Location Add Inventory Tests

Check that adding inventory to a location works, and adds the appropriate amount, in the MyStore.Store.Tests LocationTests.cs file,

Testing the MyStore.Store Location AddInventory method.
Specifically,

- can't add a negative amount of stock, should throw ArgumentOutOfRangeException

- Can't add stock to an item that doesn't exist in the model, should throw ItemNotFoundException,

- can add stock to an item that the location already has some stock of, and doesn't have stock of.