pytest: fix test_onchain_unwatch under valgrind.

valgrind slowed things down so the HTLC was resolved, so we didn't use
an anchor.  Make that case consistent.

```
    def test_onchain_unwatch(node_factory, bitcoind, chainparams):
        """Onchaind should not watch random spends"""
        # We track channel balances, to verify that accounting is ok.
        coin_mvt_plugin = os.path.join(os.getcwd(), 'tests/plugins/coin_movements.py')
        l1, l2 = node_factory.line_graph(2, opts={'plugin': coin_mvt_plugin})
        channel_id = first_channel_id(l1, l2)
    
        l1.pay(l2, 200000000)
    
        l1.rpc.dev_fail(l2.info['id'])
        l1.daemon.wait_for_log('Failing due to dev-fail command')
        l1.wait_for_channel_onchain(l2.info['id'])
    
        # Make sure we see anchor too
>       l1.bitcoin.generate_block(1, wait_for_mempool=2)

tests/test_closing.py:1769: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
contrib/pyln-testing/pyln/testing/utils.py:477: in generate_block
    wait_for(lambda: len(self.rpc.getrawmempool()) >= wait_for_mempool)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

success = <function BitcoinD.generate_block.<locals>.<lambda> at 0x7f23f909c310>
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 BitcoinD.generate_block.<locals>.<lambda> at 0x7f23f909c310>
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

tests/test_closing.py

@@ -1759,12 +1759,16 @@ def test_onchain_unwatch(node_factory, bitcoind, chainparams):
     channel_id = first_channel_id(l1, l2)
 
     l1.pay(l2, 200000000)
+    # If the HTLC isn't completely removed, we will use an anchor to bump
+    # the commitment tx.  Under valgrind we tend to resolve the HTLC
+    # before getting to dev_fail.  Unify the cases by waiting a bit.
+    wait_for(lambda: only_one(l1.rpc.listpeerchannels(l2.info['id'])['channels'])['htlcs'] == [])
 
     l1.rpc.dev_fail(l2.info['id'])
     l1.daemon.wait_for_log('Failing due to dev-fail command')
     l1.wait_for_channel_onchain(l2.info['id'])
 
-    l1.bitcoin.generate_block(1)
+    l1.bitcoin.generate_block(1, wait_for_mempool=1)
     l1.daemon.wait_for_log(' to ONCHAIN')
     l2.daemon.wait_for_log(' to ONCHAIN')
 
@@ -1785,11 +1789,7 @@ def test_onchain_unwatch(node_factory, bitcoind, chainparams):
     l1.rpc.withdraw(l1.rpc.newaddr()['bech32'], 'all')
     bitcoind.generate_block(1)
 
-    # We see *two* of these: one for anchor spend as well!
-    if chainparams['elements']:
-        l1.daemon.wait_for_log("but we don't care")
-    else:
-        l1.daemon.wait_for_logs(["but we don't care"] * 2)
+    l1.daemon.wait_for_log("but we don't care")
 
     # And lightningd should respect that!
     assert not l1.daemon.is_in_log("Can't unwatch txid")