Improve GameObjectContainer's testsuite

[nbraud]

The current testsuite for GameObjectContainer

• doesn't consider many different interleaving of operations (add, remove, ...);
• doesn't test very well the effect of operation, i.e.
  • that an object should be available after adding it, through all, get(kind) and get(tag),
  • that an object should be unreachable from the container after being removed, and
  • that an add or remove operation should leave all other objects as-is.