Allow multiple entities to be removed per attack

`table.remove` wasn't performing the necessary operation.
Update incorrect comment in entity.lua
Create more diverse testing area

Add a rectangle to the background of the scene for context.
Add a Comrade instance to the game

The desmond object can issue orders to the comrade object when a right-click is
detected.
Fix unit test broken after latest change

The problem was that the right mouse button now performs an action while in the
past, it was a no-op.
Improve Comrade movement
Improve Comrade unit test
Minor tweaks to resolution and Desmond speed