channeldb: fix data race in TestSerializeRevocationLog

When testing serializing revocation log, we need to also copy its
`HTLCEntries` as the serialization of the HTLC involves a writing to the
`htlc.amtTlv` field.

docs/release-notes/release-notes-0.15.0.md

@@ -283,6 +283,9 @@ from occurring that would result in an erroneous force close.](https://github.co
 * [Minor fix](https://github.com/lightningnetwork/lnd/pull/6535) to
   how bitcoind.rpccookie and bitocind.config are parsed from config file.
 
+* [Fix a data race found when running unit test for revocation log](https://github.com/lightningnetwork/lnd/pull/6594).
+
+
 ## RPC Server
 
 * [Add value to the field