tests: de-flake test that was failing on cltv expiry

make the number of blocks mined father away from the cltv timeout

from borked/flakey test run:

	lightningd-3 2023-01-26T21:45:19.261Z DEBUG   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-channeld-chan#2: billboard perm: Received error channel 27a4a4dd880e86
	1e390517de3e786a237c5ad1f00faab277382664e76b5c3870: Fulfilled HTLC 0 SENT_REMOVE_COMMIT cltv 116 hit deadline

tests/test_closing.py

@@ -957,7 +957,7 @@ def test_channel_lease_unilat_closes(node_factory, bitcoind):
     inv = l2.rpc.invoice(10**4, '3', 'no_3')
     l3.rpc.pay(inv['bolt11'])
 
-    bitcoind.generate_block(6)
+    bitcoind.generate_block(2)
     sync_blockheight(bitcoind, [l1, l2, l3])
     # make sure we're at the right place for the csv lock
     l2.daemon.wait_for_log('Blockheight: SENT_ADD_ACK_COMMIT->RCVD_ADD_ACK_REVOCATION LOCAL now 110')