Offer payments integration tests are flaky

[t-bast]

Some tests in the OfferPaymentSpec.scala file randomly fail during CI (for example send fully blinded multi-part payment a->b->c (single channel a->b)). They fail in the initial steps, where we wait for nodes to receive remote channel updates to be able to correctly build blinded paths and find routes to remote introduction nodes.

I'm not sure why gossip is failing like that, and it should be investigated. One possibility is that the knownFundingTxs hook has a race condition which makes channel validation fail (in Validation.scala). Another possibility is that the channel_update isn't emitted at the right time, and isn't staggered rebroadcast to the remote test nodes.