lnd_test: prevent calling Fatal in goroutine
lnd_test: prevent calling Fatal in goroutine

This commit prevents an error that I've seen on travis,
wherein the test fails because a call to Fatal happens
after the test finishes. The root cause is that we call
Fatal in a goroutine that is reading from the subscribe
graph rpc call.

To fix this, we now pass an err chan back into the main
test context, where we can receive any errors and fail
the test if one comes through.

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