multi: fix and test v1 output spend ntfns

Because Taproot key spend only spends don't allow us to re-construct the
spent pkScript from the witness alone, we cannot support registering
spend notifications for v1 pkScripts only. We instead require the
outpoint to be specified. This commit makes it possible to only match by
outpoint and also adds an itest for it.

lnrpc/chainrpc/chainnotifier.pb.go

@@ -372,7 +372,11 @@ type SpendRequest struct {
 	//
 	//The outpoint for which we should request a spend notification for. If set to
 	//a zero outpoint, then the spend notification will be requested for the
-	//script instead.
+	//script instead. A zero or nil outpoint is not supported for Taproot spends
+	//because the output script cannot reliably be computed from the witness alone
+	//and the spent output script is not always available in the rescan context.
+	//So an outpoint must _always_ be specified when registering a spend
+	//notification for a Taproot output.
 	Outpoint *Outpoint `protobuf:"bytes,1,opt,name=outpoint,proto3" json:"outpoint,omitempty"`
 	//
 	//The output script for the outpoint above. This will be used by light clients