macaroons test: reverse macaroon time caveat check
macaroons test: reverse macaroon time caveat check

This commit fixes a go 1.15 vet check.

In doing so it uncovers that the time caveat check is actually reversed.
Since we should check that the caveat is added, we should only fail the
check when the caveat prefix is not equal.

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