Add more positions for selftest/perft

[alex65536]

It would be better if there are more tests for chess rules implementations.

Possible candidates for inclusion:

• https://www.chessprogramming.org/Perft_Results
• Positions from perft/hperft part of this project (same as in https://github.com/jordanbray/chess_perft)

Probably, perft testing should be a separate part of testing the engine and not included into boards.fen. Also they can be incorporated into boards.fen (+ maybe positions generated from them on a fixed depth) and tested using regular selftest framework.