tests: promote method to utility file

We'll reuse it later

tests/test_closing.py

@@ -6,7 +6,7 @@ from pyln.testing.utils import SLOW_MACHINE
 from utils import (
     only_one, sync_blockheight, wait_for, TIMEOUT,
     account_balance, first_channel_id, closing_fee, TEST_NETWORK,
-    scriptpubkey_addr
+    scriptpubkey_addr, calc_lease_fee
 )
 
 import os
@@ -718,14 +718,6 @@ def test_penalty_outhtlc(node_factory, bitcoind, executor, chainparams):
     assert account_balance(l2, channel_id) == 0
 
 
-# check that the fee paid is correct
-def calc_lease_fee(amt, feerate, rates):
-    fee = rates['lease_fee_base_msat']
-    fee += amt * rates['lease_fee_basis'] // 10
-    fee += rates['funding_weight'] * feerate
-    return fee
-
-
 @unittest.skipIf(TEST_NETWORK != 'regtest', 'elementsd doesnt yet support PSBT features we need')
 @pytest.mark.openchannel('v2')
 @pytest.mark.slow_test

tests/test_opening.py

@@ -2,7 +2,7 @@ from fixtures import *  # noqa: F401,F403
 from fixtures import TEST_NETWORK
 from pyln.client import RpcError, Millisatoshi
 from utils import (
-    only_one, wait_for, sync_blockheight, first_channel_id
+    only_one, wait_for, sync_blockheight, first_channel_id, calc_lease_fee
 )
 
 import pytest

tests/utils.py

@@ -74,6 +74,13 @@ def move_matches(exp, mv):
     return True
 
 
+def calc_lease_fee(amt, feerate, rates):
+    fee = rates['lease_fee_base_msat']
+    fee += amt * rates['lease_fee_basis'] // 10
+    fee += rates['funding_weight'] * feerate
+    return fee
+
+
 def check_coin_moves(n, account_id, expected_moves, chainparams):
     moves = n.rpc.call('listcoinmoves_plugin')['coin_moves']
     node_id = n.info['id']