Quantum Chess: Fix en passant

[dstrain115]

En passant tests cannot be run with real devices:
https://github.com/quantumlib/ReCirq/blob/master/recirq/quantum_chess/quantum_board_test.py#L777

Note that these tests are not marked with the pytest parameterize for ALL_CIRQ_BOARDS since they cannot be mapped properly to a grid device. We should figure out a good way to fix "en passant" so that it involves only adjacent qubits in a grid layout.

See #48 for a similar problem.