lntest+itest: add method AssertNumTxsInMempool and AssertTxInBlock

in harness

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

itest/lnd_watchtower_test.go

@@ -502,7 +502,7 @@ func testRevokedCloseRetributionAltruistWatchtowerCase(ht *lntest.HarnessTest,
 	block := ht.MineBlocksAndAssertNumTxes(1, 1)[0]
 
 	breachTXID := ht.WaitForChannelCloseEvent(closeUpdates)
-	ht.Miner.AssertTxInBlock(block, breachTXID)
+	ht.AssertTxInBlock(block, breachTXID)
 
 	// The breachTXID should match the above closeTxID.
 	require.EqualValues(ht, breachTXID, closeTxID)
@@ -510,7 +510,7 @@ func testRevokedCloseRetributionAltruistWatchtowerCase(ht *lntest.HarnessTest,
 	// Query the mempool for Dave's justice transaction, this should be
 	// broadcast as Carol's contract breaching transaction gets confirmed
 	// above.
-	justiceTXID := ht.Miner.AssertNumTxsInMempool(1)[0]
+	justiceTXID := ht.AssertNumTxsInMempool(1)[0]
 
 	// Query for the mempool transaction found above. Then assert that all
 	// the inputs of this transaction are spending outputs generated by