[perft] Print the depth of the position being checked in check-positions

perft/scripts/check-positions

@@ -58,6 +58,7 @@ def main(argv):
 
         print('---')
         print(f'fen={fen}')
+        print(f'depth={depth}')
         print(f'expected-nodes={expected_nodes_count}')
 
         nodes_count = run_perft(fen, depth, expected_nodes_count)