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`.
This commit adds a new component, `HarnessTest`, as a test manager, which
is responsible for managing the state change in the itest. It is built
on top of `HarnessNode` and will be handling assertions so that a test
can be created without unnecessary attention to node's unwanted
failures. This commit also adds a minimal set of assertions.
