test: Use int.to_bytes over struct packing

This is done in prepration for the scripted diff, which can not deal
with those lines.

test/functional/wallet_balance.py

@@ -4,7 +4,6 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 """Test the wallet balance RPC methods."""
 from decimal import Decimal
-import struct
 
 from test_framework.address import ADDRESS_BCRT1_UNSPENDABLE as ADDRESS_WATCHONLY
 from test_framework.blocktools import COINBASE_MATURITY
@@ -266,8 +265,8 @@ class WalletTest(BitcoinTestFramework):
         tx_orig = self.nodes[0].gettransaction(txid)['hex']
         # Increase fee by 1 coin
         tx_replace = tx_orig.replace(
-            struct.pack("<q", 99 * 10**8).hex(),
-            struct.pack("<q", 98 * 10**8).hex(),
+            (99 * 10**8).to_bytes(8, "little", signed=True).hex(),
+            (98 * 10**8).to_bytes(8, "little", signed=True).hex(),
         )
         tx_replace = self.nodes[0].signrawtransactionwithwallet(tx_replace)['hex']
         # Total balance is given by the sum of outputs of the tx