In this commit, we introduce two mining methods, `MineBlocks` and
`MineBlocksAndAssertNumTxes`, to `HarnessTest`. These methods are
different from the miner's methods as they would 1) mine the blocks
slowly and 2) assert all active nodes have synced. The miner's methods
are kept for future tests such as mining blocks really fast and see the
behavior of `lnd`.