Speed up knapsack_solver_test by not recreating wallet 100 times.
Speed up knapsack_solver_test by not recreating wallet 100 times.

Moved the code for creating the wallet out of the 100-times repetition loop, for the most time-consuming tests.

chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin