During recent work on threat inputs, there was a hard-to-debug crash in tbprobe
caused by incorrectly computing st->materialKey. This PR adds correctness
checking to pos_is_ok and a faster check in tbprobe that will actually run in
CI (because pos_is_ok checking is disabled even in debug mode, for performance
purposes).
closes https://github.com/official-stockfish/Stockfish/pull/6410
No functional change
