slow_test confirmation_height.prioritize_frontiers_overwrite fails consistently

[dsiganos]

[ RUN      ] confirmation_height.prioritize_frontiers_overwrite

/home/dsiganos/branches/develop/nano-node/nano/slow_test/node.cpp:1141: Failure
Expected equality of these values:
  node->active.priority_cementable_frontiers_size () + node->active.priority_wallet_cementable_frontiers_size ()
    Which is: 50
  num_accounts
    Which is: 200000
[  FAILED  ] confirmation_height.prioritize_frontiers_overwrite (2353041 ms)