The recent changes to mempool inv logic mean that nodes are much less
likely to immediately return an `inv` message in response to a `mempool`
message. The `p2p_txexpiringsoon` RPC test was relying on the prior
behaviour.
`TestNode.sync_with_ping` now takes an optional `waiting_for` closure
that allows the caller to require that a specific message kind is
received prior to the timeout.
Jack Grigg
mdr0id
Daira Hopwood
Eirik Ogilvie-Wigley