lntest: wait for graph subscription on node init
lntest: wait for graph subscription on node init

During the channel_backup_restore/restore_during_unlock itest, the node
is restored from seed and immediately restarted. Depending on specific
timing of the machine, the test harness might not have had the graph
subscription processed before the node shuts down, causing the harness
to trigger a panic.

Reducing this to a synchronous subscription attempt means node
initialization necessarily waits until the subscription is done before
attempting to restart, reducing flakiness and ensuring correct behavior.

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