rpctest: increase sweeper BatchWindow during itests

Since we are checking HTLC aggregation, we must give the sweeper a bit
more time to aggregate them to avoid flakes.

sweep/defaults_rpctest.go

@@ -13,5 +13,5 @@ var (
 	//
 	// To speed up integration tests waiting for a sweep to happen, the
 	// batch window is shortened.
-	DefaultBatchWindowDuration = 2 * time.Second
+	DefaultBatchWindowDuration = 8 * time.Second
 )