lntest+itest: add method AssertNumTxsInMempool and AssertTxInBlock

in harness

Prepare to make `HarnessTest.Miner` a private instance to sync height.

itest/lnd_open_channel_test.go

@@ -45,7 +45,7 @@ func testOpenChannelAfterReorg(ht *lntest.HarnessTest) {
 
 	// Wait for miner to have seen the funding tx. The temporary miner is
 	// disconnected, and won't see the transaction.
-	ht.Miner.AssertNumTxsInMempool(1)
+	ht.AssertNumTxsInMempool(1)
 
 	// At this point, the channel's funding transaction will have been
 	// broadcast, but not confirmed, and the channel should be pending.
@@ -59,7 +59,7 @@ func testOpenChannelAfterReorg(ht *lntest.HarnessTest) {
 	// channel on the original miner's chain, which should be considered
 	// open.
 	block := ht.MineBlocksAndAssertNumTxes(10, 1)[0]
-	ht.Miner.AssertTxInBlock(block, fundingTxID)
+	ht.AssertTxInBlock(block, fundingTxID)
 	_, err = tempMiner.Client.Generate(15)
 	require.NoError(ht, err, "unable to generate blocks")
 
@@ -115,7 +115,7 @@ func testOpenChannelAfterReorg(ht *lntest.HarnessTest) {
 
 	// Cleanup by mining the funding tx again, then closing the channel.
 	block = ht.MineBlocksAndAssertNumTxes(1, 1)[0]
-	ht.Miner.AssertTxInBlock(block, fundingTxID)
+	ht.AssertTxInBlock(block, fundingTxID)
 
 	ht.CloseChannel(alice, chanPoint)
 }