itest: Fix flakes when payments cause chan closure
itest: Fix flakes when payments cause chan closure

This fixes itest flakes that happen when a payment is attempted that
ends up causing a channel closure.

completePaymentRequests() attempts to monitor the open channels after a
payment is attempted in order to identify that payment was actually
dispatched to a remote node before returning.

However, when the payment actually causes a channel closure (for
example, because the receiver sent an incorrect preimage) this logic
fails in that the channel will no longer the found in the list of open
channels. This could cause a flake when there was enough time for the
channel to close before performing the check.

One example of such a flaky test is failing_link.

This fixes the issue by also checking whether the total number of
channels was reduced, which indicates (assuming itest operations are
being executed serially) that one of the attempted payments affected at
least one channel.

LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd