Make BoardSpace validation less flaky

[munichpavel]

For example, this is valid (from tests/test_state.py, commit 00fc7a1

to_space=BoardSpace(
    kind='main', idx=1,
    occupied_by=EMPTY_SYMBOL,
    allowed_occupants=self.player_names + [EMPTY_SYMBOL]
)

but this is not

to_space=BoardSpace(
    kind='main', idx=1,
    occupied_by=EMPTY_SYMBOL,
    allowed_occupants=[EMPTY_SYMBOL] + self.player_names
)

as allowed_occupants check is a simple test of equality of lists, so order matters.