chainntnfs/interface_test: increase wait time for mempool spends
chainntnfs/interface_test: increase wait time for mempool spends

This commit increases the time we wait for a spend client to notify a
mempool spend from 50ms to 10s. This is done to catch the case where
bitcoind would use up to 7 seconds before notifying about a mempool
spend, which wasn't caught by the test.

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