Implementing tests for en passant capture.
Implemented test for en passant capture when the piece to be captured is not in the en passant array. En passant capture is now being handled correctly by the pawn evaluator.
Added test to ensure that en passant-eligible pawns are being correctly tracked. Tweaked en passant logic so tests pass.
Added tests asserting that en passant capture is performed correctly at the evaluate_move level. Tests are passing. En passant capture appears to be implemented.