pytest: slow down for bitcoind.

Failure under CI:

```
>       bitcoind.generate_block(1000)

tests/test_closing.py:853: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
contrib/pyln-testing/pyln/testing/utils.py:496: in generate_block
    return self.rpc.generatetoaddress(numblocks, to_addr)
contrib/pyln-testing/pyln/testing/utils.py:374: in f
    res = proxy._call(name, *args)
../../../.cache/pypoetry/virtualenvs/cln-meta-project-AqJ9wMix-py3.7/lib/python3.7/site-packages/bitcoin/rpc.py:246: in _call
    response = self._get_response()
../../../.cache/pypoetry/virtualenvs/cln-meta-project-AqJ9wMix-py3.7/lib/python3.7/site-packages/bitcoin/rpc.py:276: in _get_response
    http_response = self.__conn.getresponse()
/opt/hostedtoolcache/Python/3.7.16/x64/lib/python3.7/http/client.py:1373: in getresponse
    response.begin()
/opt/hostedtoolcache/Python/3.7.16/x64/lib/python3.7/http/client.py:319: in begin
    version, status, reason = self._read_status()
/opt/hostedtoolcache/Python/3.7.16/x64/lib/python3.7/http/client.py:280: in _read_status
    line = str(self.fp.readline(_MAXLINE + 1), "iso-8859-1")
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <socket.SocketIO object at 0x7fa21aa5d710>
b = <memory at 0x7fa21b771390>

    def readinto(self, b):
        """Read up to len(b) bytes into the writable buffer *b* and return
        the number of bytes read.  If the socket is non-blocking and no bytes
        are available, None is returned.
    
        If *b* is non-empty, a 0 return value indicates that the connection
        was shutdown at the other end.
        """
        self._checkClosed()
        self._checkReadable()
        if self._timeout_occurred:
            raise OSError("cannot read from timed out object")
        while True:
            try:
>               return self._sock.recv_into(b)
E               socket.timeout: timed out

/opt/hostedtoolcache/Python/3.7.16/x64/lib/python3.7/socket.py:589: timeout
```

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

tests/test_closing.py

@@ -842,21 +842,11 @@ def test_channel_lease_post_expiry(node_factory, bitcoind, chainparams):
     l2.daemon.wait_for_log('Blockheight: SENT_ADD_ACK_COMMIT->RCVD_ADD_ACK_REVOCATION LOCAL now 115')
 
     # We need to give l1-l2 time to update their blockheights
-    bitcoind.generate_block(1000)
-    sync_blockheight(bitcoind, [l1, l2])
-    l1.daemon.wait_for_log('peer_out WIRE_UPDATE_BLOCKHEIGHT')
-
-    bitcoind.generate_block(1000)
-    sync_blockheight(bitcoind, [l1, l2])
-    l1.daemon.wait_for_log('peer_out WIRE_UPDATE_BLOCKHEIGHT')
-
-    bitcoind.generate_block(1000)
-    sync_blockheight(bitcoind, [l1, l2])
-    l1.daemon.wait_for_log('peer_out WIRE_UPDATE_BLOCKHEIGHT')
-
-    bitcoind.generate_block(1000)
-    sync_blockheight(bitcoind, [l1, l2])
-    l1.daemon.wait_for_log('peer_out WIRE_UPDATE_BLOCKHEIGHT')
+    for i in range(0, 4000, 1000):
+        for _ in range(0, 1000, 200):
+            bitcoind.generate_block(200)
+        sync_blockheight(bitcoind, [l1, l2])
+        l1.daemon.wait_for_log('peer_out WIRE_UPDATE_BLOCKHEIGHT')
 
     bitcoind.generate_block(32)
     sync_blockheight(bitcoind, [l1, l2])