Add tests for transitions

[silviot]

These tests rely on the existing object definitions.

The following transition, defined in the test, should probably be added to the existing ones.

('last', 2, 3): {
                'actor_end': 5,
                'actor_start': 2,
                'last_use': False,
                'modify_uses': [0, 0],
                'target_end': 3,
                'target_start': 3,
                'visible': 'always'
            },

[alecpm]

I think we still need to test when to use the regular vs last transition here, as well as the modify_uses stuff, but this is a good start.

[alecpm]

Since you're using this fixture twice, maybe we move it out to the module level (or into conftest.py)?

[alecpm]

There will be an issue with this once @jessesnyder merges PR #252 which updates the game_config.yml to use string ids to make these transition definitions a little less opaque. If we're going to depend on the existing game_config.yml maybe this could just test the existing Gooseberry Bush -> Empty Gooseberry Bush transition last_use transition?