lnd_test: use child harness chan backup restore cases
lnd_test: use child harness chan backup restore cases

This prevents a panic during test failure due to a child test calling
FailNow on a parent test context. The sub tests now capture the
testing.T object provided in each closure as opposed to ignoring it and
using the parent context.

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