itest+lntest: add new method FundNumCoins

Most of the time we only need to fund the node with given number of
UTXOs without concerning the amount, so we add the more efficient
funding method as it mines a single block in the end.

lntest/harness.go

@@ -1379,6 +1379,40 @@ func (h *HarnessTest) FundCoinsP2TR(amt btcutil.Amount,
 	h.fundCoins(amt, target, lnrpc.AddressType_TAPROOT_PUBKEY, true)
 }
 
+// FundNumCoins attempts to send the given number of UTXOs from the internal
+// mining node to the targeted lightning node using a P2WKH address. Each UTXO
+// has an amount of 1 BTC. 1 blocks are mined to confirm the tx.
+func (h *HarnessTest) FundNumCoins(hn *node.HarnessNode, num int) {
+	// Get the initial balance first.
+	resp := hn.RPC.WalletBalance()
+	initialBalance := btcutil.Amount(resp.ConfirmedBalance)
+
+	const fundAmount = 1 * btcutil.SatoshiPerBitcoin
+
+	// Send out the outputs from the miner.
+	for i := 0; i < num; i++ {
+		h.createAndSendOutput(
+			hn, fundAmount, lnrpc.AddressType_WITNESS_PUBKEY_HASH,
+		)
+	}
+
+	// Wait for ListUnspent to show the correct number of unconfirmed
+	// UTXOs.
+	//
+	// Since neutrino doesn't support unconfirmed outputs, skip this check.
+	if !h.IsNeutrinoBackend() {
+		h.AssertNumUTXOsUnconfirmed(hn, num)
+	}
+
+	// Mine a block to confirm the transactions.
+	h.MineBlocksAndAssertNumTxes(1, num)
+
+	// Now block until the wallet have fully synced up.
+	totalAmount := btcutil.Amount(fundAmount * num)
+	expectedBalance := initialBalance + totalAmount
+	h.WaitForBalanceConfirmed(hn, expectedBalance)
+}
+
 // completePaymentRequestsAssertStatus sends payments from a node to complete
 // all payment requests. This function does not return until all payments
 // have reached the specified status.