chainntnfs/interface_test: fix unreliable historical block ntfns test
chainntnfs/interface_test: fix unreliable historical block ntfns test

After joining the two forked chains, it is necessary to ensure they both agree on the same best hash before proceeding to UnsafeStart the notifier.
This is because when the BitcoindClient starts, it retrieves its best known block then calls GetBlockHeaderVerbose on the hash of the retrieved block. This block could be a reorged block if JoinNodes has not completed sync. If it is the case that the best block retrieved has been reorged out of the chain, GetBlockHeaderVerbose errors because bitcoind sets the number of confirmations to -1 on reorged blocks, and the btcd rpc client panics when parsing a block whose number of confirmations is negative.

This parsing error is expected to be fixed, and as a more permanent solution chain backends should ensure that the `best block` they retrieve during startup has not been reorged out of the chain.

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