The method `AssertActiveHtlcs` is now removed due to it's easy to be
misused. To assert a given htlc, use `AssertOutgoingHTLCActive` and
`AssertIncomingHTLCActive` instead for ensuring the HTLC exists in the
right direction. Although often the case `AssertNumActiveHtlcs` would be
enough as it implicitly checks the forwarding behavior for an
intermediate node by asserting there are always num_payment*2 HTLCs.
Starting from this commit, we begin the process of flattening the
multi-hop itests to make them easier to be maintained. The tests are
refactored into their own test cases, with each test focusing on testing
one channel type. This is necessary to save effort for future
development.
These tests are also updated to reflect the new `blockbeat` behavior.
