Slow down some TOO FAST tests in backend

- Product properties
- Option Types
- Promotions

All these tests were attempting to run code on elements on the page which hadn't loaded. Gotta sloooowwww down and wait for the elements to become available.
No need for tap within a couple InventoryUnit tests
Grab the real first stock_item in InventoryUnit spec
We've already got the first stock item in stock_items_controller spec. Let's use it
[api] Assign first stock_item to let block within StockMovementsController spec and use it throughout