htlcswitch/decayedlog_test: fix bug in gc unit test
htlcswitch/decayedlog_test: fix bug in gc unit test

This commit fixes a bug in the
TestDecayedLogPersistentGarbageCollector unit test.
The test generates a second hash prefix, which is never
added to the log, and used to query for the final
existence check. This commit reverts the behavior so
that the same hash prefix is used throughout the test.

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