This commit breaks the large `testMultiHopHtlcClaims` into independent
test cases to take the advantage of parallel testing. The old setup will
put the stress of creating and syncing new blocks into a single core,
which causes a much longer test time and sometimes times out.
This commit adds a new struct, `HarnessNode`, to manage the lnd process
used in our itest. This struct is built upon `HarnessRPC`, `State`, and
`NodeWatcher`.
