Unit tests

[AdamStelmaszczyk]

I was looking for unit tests to understand corner cases + see how many of them are to port to C++. I haven't found any unit test of pommerman forward model logic.

It will be good to have at least regression tests when fixing bugs like: b16c395, 3c06283, 8b57431.

[cinjon]

Sigh. It's true. We've been very lax on that and haven't put in the effort to build all the proper unit tests. This would be very helpful as a contribution and probably necessary for #103 .

[suphoff]

Once #123 is resolved recorded (state x action -> state) steps can be repeated by different implementations of step functions and compared with the expected state.