lnd_test: invoice subscription cancel lingering goroutine
lnd_test: invoice subscription cancel lingering goroutine

This commit adds a small cancellation check to the
invoice subscription integration test. Calls to
Fatalf were observed on travis after the test had
ended stemming from the subscriber's goroutine,
which could happen if the subscription is late.
It also extends the timeout on the test from 5 to
10 seconds.

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