lntest: re-define CleanupForceClose

To reflect the new sweeping behavior, also makes it easier to be used as
we need to method to quickly cleanup force closes without concerning the
details when we are not testing the force close behavior.

lntest/harness_miner.go

@@ -167,16 +167,9 @@ func (h *HarnessTest) cleanMempool() {
 }
 
 // mineTillForceCloseResolved asserts that the number of pending close channels
-// are zero. Each time it checks, a new block is mined using MineBlocksSlow to
-// give the node some time to catch up the chain.
-//
-// NOTE: this method is a workaround to make sure we have a clean mempool at
-// the end of a channel force closure. We cannot directly mine blocks and
-// assert channels being fully closed because the subsystems in lnd don't share
-// the same block height. This is especially the case when blocks are produced
-// too fast.
-// TODO(yy): remove this workaround when syncing blocks are unified in all the
-// subsystems.
+// are zero. Each time it checks, an empty block is mined, followed by a
+// mempool check to see if there are any sweeping txns. If found, these txns
+// are then mined to clean up the mempool.
 func (h *HarnessTest) mineTillForceCloseResolved(hn *node.HarnessNode) {
 	_, startHeight := h.GetBestBlock()
 
@@ -184,7 +177,15 @@ func (h *HarnessTest) mineTillForceCloseResolved(hn *node.HarnessNode) {
 		resp := hn.RPC.PendingChannels()
 		total := len(resp.PendingForceClosingChannels)
 		if total != 0 {
-			h.MineBlocks(1)
+			// Mine an empty block first.
+			h.MineEmptyBlocks(1)
+
+			// If there are new sweeping txns, mine a block to
+			// confirm it.
+			mem := h.GetRawMempool()
+			if len(mem) != 0 {
+				h.MineBlocksAndAssertNumTxes(1, len(mem))
+			}
 
 			return fmt.Errorf("expected num of pending force " +
 				"close channel to be zero")