Extensify tests

[EtienneBruines]

Would be nice to have a test for every possible aspect, to ensure it's stable.

[EtienneBruines]

What is there left to test?

@paked

[paked]

We should probably look at getting some world-level integration tests happening.

[Noofbiz]

I'll also add some tests for race conditions on the world and entities. I wanna make this library thread-safe as well