tests: simplify next_block() function in feature_block
tests: simplify next_block() function in feature_block

The solve parameter is unnecessary. Remove it and add
comments.


Former-commit-id: e114e07e5978c71fcf73b89a80752ecbe749f1c6
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin