itest: fix misuse of MineBlocks and replace it with

`MineBlocksAndAssertNumTxes`

itest/lnd_mpp_test.go

@@ -208,7 +208,8 @@ func newMppTestScenario(ht *lntest.HarnessTest) *mppTestScenario {
 		ht.FundCoinsUnconfirmed(btcutil.SatoshiPerBitcoin, carol)
 		ht.FundCoinsUnconfirmed(btcutil.SatoshiPerBitcoin, dave)
 		ht.FundCoinsUnconfirmed(btcutil.SatoshiPerBitcoin, eve)
-		ht.MineBlocks(1)
+
+		ht.MineBlocksAndAssertNumTxes(1, 3)
 	}
 
 	mts := &mppTestScenario{
@@ -318,7 +319,7 @@ func (m *mppTestScenario) closeChannels() {
 	m.ht.CloseChannelAssertPending(m.eve, m.channelPoints[5], false)
 
 	// Now mine a block to include all the closing transactions.
-	m.ht.MineBlocks(1)
+	m.ht.MineBlocksAndAssertNumTxes(1, 6)
 
 	// Assert that the channels are closed.
 	for _, hn := range m.nodes {