pytest: fix flake in test_anchorspend_using_to_remote

[rustyrussell]

With recent changes, particularly using postgres, we can finish the HTLC before we force close, so the test fails. Insert an explicit hangup here so we reliably get the result we expect:

    @pytest.mark.parametrize("anchors", [False, True])
    @unittest.skipIf(TEST_NETWORK != 'regtest', 'elementsd anchors not supportd')
    def test_anchorspend_using_to_remote(node_factory, bitcoind, anchors):
        """Make sure we can use `to_remote` output of previous close to spend anchor"""
        # Try with old output from both anchor and non-anchor channel.
        l4_opts = {}
        if anchors is False:
            l4_opts['dev-force-features'] = "-23"
    
        l1, l2, l3, l4 = node_factory.get_nodes(4, opts=[{},
                                                         {},
                                                         {'disconnect': ['-WIRE_UPDATE_FULFILL_HTLC']},
                                                         l4_opts])
    
        # Give l2 some funds, from a to-remote output.  It will have to spend
        # this to use anchor.
        node_factory.join_nodes([l4, l2])
    
        # l4 unilaterally closes, l2 gets to-remote with its output.
        l4.rpc.pay(l2.rpc.invoice(100000000, 'test', 'test')['bolt11'])
>       wait_for(lambda: only_one(l4.rpc.listpeerchannels()['channels'])['htlcs'] != [])

tests/test_closing.py:4183: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

success = <function test_anchorspend_using_to_remote.<locals>.<lambda> at 0x7f8293e7fe20>
timeout = 180

    def wait_for(success, timeout=TIMEOUT):
        start_time = time.time()
        interval = 0.25
        while not success():
            time_left = start_time + timeout - time.time()
            if time_left <= 0:
>               raise ValueError("Timeout while waiting for {}".format(success))
E               ValueError: Timeout while waiting for <function test_anchorspend_using_to_remote.<locals>.<lambda> at 0x7f8293e7fe20>

Changelog-None: flake fixes

[endothermicdev]

ACK 0dc33f0