test: eliminate sleep by tightly polling for existence of node
test: eliminate sleep by tightly polling for existence of node

This commit elminates a sleep in the testHtlcErrorPropagation test by
instead polling for the existence of our target in a tight loop. This
is a small patch over until we get a “topology client” within the
ChannelRouter. This method is robust and can be applied in future tests
until we get the notification client into the ChannelRouter.

AndrewSamokhvalov/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,aakselrod/lnd,ddustin/lnd,LightningNetwork/lnd,ddustin/lnd,LightningNetwork/lnd