lnd_test: shutdown nodes using defers
lnd_test: shutdown nodes using defers

This commit adds the helper method shutdownAndAssert, and makes us shut
down nodes created temporarily during the test cases using defers
instead of at the end of the test. This makes it easier to remember to
shut them down, as it is done right after they are started.

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