For dark hex, observation tensors with "reveal-numturns" were erroring on games of max length. Fixed by adding 1 to the observation tensor length.

Also, infostate tensors with "reveal-numturns" were erroring on games of max length. Fixed by using the num_cells_th bit instead of num_cells_+1th bit for unknown action.

Ran the following script to generate a playthrough as a regression test. Verified that it errored on master.

python open_spiel/python/examples/playthrough.py \
--game "dark_hex(gameversion=adh,obstype=reveal-numturns)" \
--output_file open_spiel/integration_tests/playthroughs/dark_hex_reveal_turn_long.txt \
--alsologtostdout --actions "0,2,1,3,4,6,5,7,7,5,6,4,3,1,2,0,8"