pytest: fix another flake in test_zeroconf_forget.

fundwallet generates a block: if l2 sees one block and not the other, it can
consider the time on the first fundchannel to be one block earlier than expected:

```
2025-05-05T21:05:33.6260600Z E                   TimeoutError: Unable to find "[re.compile('UNUSUAL 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#1: Forgetting channel: It has been 51 blocks without the funding transaction ')]" in logs.
...
2025-05-05T21:05:33.7179461Z lightningd-2 2025-05-05T20:44:26.141Z UNUSUAL 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#1: Forgetting channel: It has been 52 blocks without the funding transaction 5a18d113f53df8b205ab679924babde4068f2d1876d34edc43701bf92b8ff13f getting deeply confirmed. We are fundee and can forget channel without loss of funds.
```

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

Author: rustyrussell

tests/test_opening.py

@@ -2720,10 +2720,10 @@ def censoring_sendrawtx(tx):
 
     l1.fundwallet(10**7)
     l3.fundwallet(10**7)
+    sync_blockheight(bitcoind, [l2])
 
     l1.connect(l2)
     l1.rpc.fundchannel(l2.info["id"], 10**6, mindepth=0)
-    sync_blockheight(bitcoind, [l1, l2])
     wait_for(lambda: l2.rpc.listincoming()["incoming"] != [])
 
     # If we are told to pay while still not confirmed we perform one