lnwallet: properly set the short chan ID for remote unilateral close summaries

In this commit, we fix an existing bug wherein we wouldn't set the short
channel ID for the close summary in the database in the case that the
remote party force closed. The fix is simple, ensure that within
NewUnilateralCloseSummary we properly set the short channel ID. A test
has also been added in this commit, which fails without the
modifications to lnwallet/channel.go.

Fixes #2072.

lnwallet/channel.go

@@ -5109,6 +5109,7 @@ func NewUnilateralCloseSummary(chanState *channeldb.OpenChannel, signer Signer,
 		IsPending:               true,
 		RemoteCurrentRevocation: chanState.RemoteCurrentRevocation,
 		RemoteNextRevocation:    chanState.RemoteNextRevocation,
+		ShortChanID:             chanState.ShortChanID(),
 		LocalChanConfig:         chanState.LocalChanCfg,
 	}