Non-deterministic test failure in quantum chess

[rmlarose]

Noticed a failure in test_undo_entangled_measurement on PR #96 which is unrelated to quantum chess. It doesn't seem to fail often but I can reproduce it after running several times.

=================================== FAILURES ===================================
_______________________ test_undo_entangled_measurement ________________________

    def test_undo_entangled_measurement():
        b = qb.CirqBoard(u.squares_to_bitboard(['a2','b1','c2','d1']))
        assert b.perform_moves(
            'b1a3c3:SPLIT_JUMP:BASIC',
            'c2c4:PAWN_TWO_STEP:BASIC'
        )
        probs = b.get_probability_distribution(1000)
        assert_prob_about(probs, qb.square_to_bit('a3'), 0.5)
        assert_prob_about(probs, qb.square_to_bit('c2'), 0.5)
        assert_prob_about(probs, qb.square_to_bit('c3'), 0.5)
        assert_prob_about(probs, qb.square_to_bit('c4'), 0.5)
        b.perform_moves( 'd1c2:JUMP:EXCLUDED')
        assert b.undo_last_move()
        print(b)
        probs = b.get_probability_distribution(1000)
>       assert_prob_about(probs, qb.square_to_bit('a3'), 0.5)

recirq/quantum_chess/quantum_board_test.py:888: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

probs = [0.0, 0.0, 0.0, 1.0, 0.0, 0.0, ...], that = 16, expected = 0.5
atol = 0.04

    def assert_prob_about(probs, that, expected, atol=0.04):
        """Checks that the probability is within atol of the expected value."""
        assert probs[that] > expected - atol
>       assert probs[that] < expected + atol
E       assert 0.54 < (0.5 + 0.04)

recirq/quantum_chess/test_utils.py:83: AssertionError
----------------------------- Captured stdout call -----------------------------
 +----------------------------------+
8|  .   .   .   .   .   .   .   .   |
7|  .   .   .   .   .   .   .   .   |
6|  .   .   .   .   .   .   .   .   |
5|  .   .   .   .   .   .   .   .   |
4|  .   .   54  .   .   .   .   .   |
3|  54  .   46  .   .   .   .   .   |
2| 100  .   46  .   .   .   .   .   |
1|  .   .   .  100  .   .   .   .   |
 +----------------------------------+
    a   b   c   d   e   f   g   h   

[mpharrigan]

@dstrain115 @cantwellc