refactor: test: use _ variable for unused loop counters

substitutes "for x in range(N):" by "for _ in range(N):"
indicates to the reader that a block is just repeated N times, and
that the loop counter is not used in the body

test/functional/wallet_descriptor.py

@@ -107,7 +107,7 @@ class WalletDescriptorTest(BitcoinTestFramework):
         assert_equal(info2['desc'], info3['desc'])
 
         self.log.info("Test that getnewaddress still works after keypool is exhausted in an encrypted wallet")
-        for i in range(0, 500):
+        for _ in range(500):
             send_wrpc.getnewaddress()
 
         self.log.info("Test that unlock is needed when deriving only hardened keys in an encrypted wallet")
@@ -120,7 +120,7 @@ class WalletDescriptorTest(BitcoinTestFramework):
         }])
         send_wrpc.walletlock()
         # Exhaust keypool of 100
-        for i in range(0, 100):
+        for _ in range(100):
             send_wrpc.getnewaddress(address_type='bech32')
         # This should now error
         assert_raises_rpc_error(-12, "Keypool ran out, please call keypoolrefill first", send_wrpc.getnewaddress, '', 'bech32')