Unit test for double entity removal

Took me a while to figure out the issue and how to reproduce it in a small setup, but here it is.
It wasn't the components, but the entities themselves that where added back to the pool unchecked.
In my case, one system was adding the entities and another was removing them, but for simplicity I put all of the code inside a single system in the test.
You can use the `addedToEngine()` method rather than passing the `Engine` in the constructor.