Add this unit test to threshold cost test

[sappenin]

 For example, if a threshold crypto-condition contains 5 sub-
   conditions with costs of 64, 64, 82, 84 and 84 and has a threshold of
   3, the cost is equal to the sum of the largest three sub-condition
   costs (82 + 84 + 84 = 250) plus 1024 times the number of sub-
   conditions (1024 * 5 = 5120): 5370