add unit tests for GetMinFee

This adds a bunch of unit tests for the fee calculation. For initial testing, I added a tests which fail with either the new or the old fee rules: GetMinFee_createFree_test passes only with the new rules while GetMinFee_createNoFree_test passes only with the old rules. Before merging, I'll remove on of these tests, but I though it's good to include both. All other tests pass with both the old rules and the new rules.