itest+lntest: add itest testSweepHTLCs to check HTLC sweepings

lntest/harness_miner.go

@@ -476,6 +476,30 @@ func (h *HarnessMiner) MineBlockWithTxes(txes []*btcutil.Tx) *wire.MsgBlock {
 	block, err := h.Client.GetBlock(b.Hash())
 	require.NoError(h, err, "unable to get block")
 
+	// Make sure the mempool has been updated.
+	for _, tx := range txes {
+		h.AssertTxNotInMempool(*tx.Hash())
+	}
+
+	return block
+}
+
+// MineBlocksWithTx mines a single block to include the specifies tx only.
+func (h *HarnessMiner) MineBlockWithTx(tx *wire.MsgTx) *wire.MsgBlock {
+	var emptyTime time.Time
+
+	txes := []*btcutil.Tx{btcutil.NewTx(tx)}
+
+	// Generate a block.
+	b, err := h.GenerateAndSubmitBlock(txes, -1, emptyTime)
+	require.NoError(h, err, "unable to mine block")
+
+	block, err := h.Client.GetBlock(b.Hash())
+	require.NoError(h, err, "unable to get block")
+
+	// Make sure the mempool has been updated.
+	h.AssertTxNotInMempool(tx.TxHash())
+
 	return block
 }