pytest: fix timeout in test_sql

The logs show we're not waiting for the right transaction, so be explicit.

```
        # Check that channels gets refreshed!
        scid = l1.get_channel_scid(l2)
        l1.rpc.setchannel(scid, feebase=123)
        wait_for(lambda: l3.rpc.sql("SELECT short_channel_id FROM channels WHERE base_fee_millisatoshi = 123;")['rows'] == [[scid]])
        l3.daemon.wait_for_log("Refreshing channels...")
        l3.daemon.wait_for_log("Refreshing channel: {}".format(scid))
    
        # This has to wait for the hold_invoice plugin to let go!
        l1.rpc.close(l2.info['id'])
        bitcoind.generate_block(13, wait_for_mempool=1)
>       wait_for(lambda: len(l3.rpc.listchannels()['channels']) == 2)

tests/test_plugin.py:4143: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

success = <function test_sql.<locals>.<lambda> at 0x7fde3b9cd3a0>, 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_sql.<locals>.<lambda> at 0x7fde3b9cd3a0>
```

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_plugin.py

@@ -4138,8 +4138,8 @@ def test_sql(node_factory, bitcoind):
     l3.daemon.wait_for_log("Refreshing channel: {}".format(scid))
 
     # This has to wait for the hold_invoice plugin to let go!
-    l1.rpc.close(l2.info['id'])
-    bitcoind.generate_block(13, wait_for_mempool=1)
+    txid = l1.rpc.close(l2.info['id'])['txid']
+    bitcoind.generate_block(13, wait_for_mempool=txid)
     wait_for(lambda: len(l3.rpc.listchannels()['channels']) == 2)
     assert len(l3.rpc.sql("SELECT * FROM channels;")['rows']) == 2
     l3.daemon.wait_for_log("Deleting channel: {}".format(scid))