mem leak in test_onchain_multihtlc_our_unilateral

In our CI I found

```
==================================== ERRORS ====================================
__________ ERROR at teardown of test_onchain_multihtlc_our_unilateral __________
[gw4] linux -- Python 3.7.15 /home/runner/.cache/pypoetry/virtualenvs/cln-meta-project-AqJ9wMix-py3.7/bin/python

request = >

@pytest.fixture
def teardown_checks(request):
"""A simple fixture to collect errors during teardown.

We need to collect the errors and raise them as the very last step in the
fixture tree, otherwise some fixtures may not be cleaned up
correctly. Require this fixture in all other fixtures that need to either
cleanup before reporting an error or want to add an error that is to be
reported.

"""
errors = TeardownErrors()
yield errors

if errors.has_errors():
# Format a nice list of everything that went wrong and raise an exception
request.node.has_errors = True
> raise ValueError(str(errors))
E ValueError:
E Node errors:
E Global errors:
E - Node /tmp/ltests-iguww9bk/test_onchain_multihtlc_our_unilateral_1/lightning-5/ has memory leaks: [
E {
E "backtrace": [
E "/home/runner/work/lightning/lightning/ccan/ccan/tal/tal.c:442 (tal_alloc_)",
E "/home/runner/work/lightning/lightning/ccan/ccan/io/io.c:91 (io_new_conn_)",
E "/home/runner/work/lightning/lightning/lightningd/subd.c:774 (new_subd)",
E "/home/runner/work/lightning/lightning/lightningd/subd.c:828 (new_channel_subd_)",
E "/home/runner/work/lightning/lightning/lightningd/onchain_control.c:651 (onchaind_funding_spent)",
E "/home/runner/work/lightning/lightning/lightningd/peer_control.c:1884 (funding_spent)",
E "/home/runner/work/lightning/lightning/lightningd/watch.c:271 (txowatch_fire)",
E "/home/runner/work/lightning/lightning/lightningd/chaintopology.c:82 (filter_block_txs)",
E "/home/runner/work/lightning/lightning/lightningd/chaintopology.c:732 (add_tip)",
E "/home/runner/work/lightning/lightning/lightningd/chaintopology.c:823 (get_new_block)",
E "/home/runner/work/lightning/lightning/lightningd/bitcoind.c:383 (getrawblockbyheight_callback)",
E "/home/runner/work/lightning/lightning/lightningd/plugin.c:590 (plugin_response_handle)",
E "/home/runner/work/lightning/lightning/lightningd/plugin.c:701 (plugin_read_json_one)",
E "/home/runner/work/lightning/lightning/lightningd/plugin.c:746 (plugin_read_json)",
E "/home/runner/work/lightning/lightning/ccan/ccan/io/io.c:59 (next_plan)",
E "/home/runner/work/lightning/lightning/ccan/ccan/io/io.c:407 (do_plan)",
E "/home/runner/work/lightning/lightning/ccan/ccan/io/io.c:417 (io_ready)",
E "/home/runner/work/lightning/lightning/ccan/ccan/io/poll.c:453 (io_loop)",
E "/home/runner/work/lightning/lightning/lightningd/io_loop_with_timers.c:22 (io_loop_with_timers)",
E "/home/runner/work/lightning/lightning/lightningd/lightningd.c:[1227](https://github.com/ElementsProject/lightning/actions/runs/3996931879/jobs/6858176075#step:7:1228) (main)"
E ],
E "label": "ccan/ccan/io/io.c:91:struct io_conn",
E "parents": [
E "lightningd/lightningd.c:106:struct lightningd"
E ],
E "value": "0x557c7d47a558"
E }
E ]

contrib/pyln-testing/pyln/testing/fixtures.py:206: ValueError
```