lntest+itest: add method AssertNumTxsInMempool and AssertTxInBlock

in harness

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

itest/lnd_signer_test.go

@@ -296,7 +296,7 @@ func assertSignOutputRaw(ht *lntest.HarnessTest,
 	alice.RPC.SendCoins(req)
 
 	// Wait until the TX is found in the mempool.
-	txid := ht.Miner.AssertNumTxsInMempool(1)[0]
+	txid := ht.AssertNumTxsInMempool(1)[0]
 
 	targetOutputIndex := ht.GetOutputIndex(txid, targetAddr.String())
 
@@ -359,7 +359,7 @@ func assertSignOutputRaw(ht *lntest.HarnessTest,
 	})
 
 	// Wait until the spending tx is found.
-	txid = ht.Miner.AssertNumTxsInMempool(1)[0]
+	txid = ht.AssertNumTxsInMempool(1)[0]
 	p2wkhOutputIndex := ht.GetOutputIndex(txid, p2wkhAdrr.String())
 
 	op := &lnrpc.OutPoint{