Before we have sufficient signaling in the network to relay this
signal, set a zero value experimental endorsement value on the sender's
outgoing htlc. Once the network is relaying this signal and a flag day
has been set, we'll be able to set a non-zero value here.
This commit replaces `AssertTopologyChannelOpen` with
`AssertChannelInGraph`, which asserts a given channel edge is found.
`AssertTopologyChannelOpen` only asserts a given edge has been received
via the topology subscription, while we need to make sure the channel is
in the graph before continuing our tests.
This commit introduces a basic integration test for the invoice
HTLC modifier. The test covers scenarios where an invoice is settled with a
payment that is less than the invoice amount, facilitated by the invoice
HTLC modifier.
