pytest: fix flake in test_closing_anchorspend_htlc_tx_rbf

```
2026-04-02T04:45:50.5882651Z >       assert 3000 - 1 < total_feerate_perkw < 3000 + 1
2026-04-02T04:45:50.5883085Z E       assert 3001.3083296990844 < (3000 + 1)
2026-04-02T04:45:50.5883354Z
2026-04-02T04:45:50.5883508Z tests/test_closing.py:3853: AssertionError
```

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

Author: rustyrussell

tests/test_closing.py

@@ -3787,7 +3787,8 @@ def test_closing_anchorspend_htlc_tx_rbf(node_factory, bitcoind):
     # We want an outstanding HTLC for l1, so it uses anchor to push.
     # Set feerates to lowball for now.
     l1, l2 = node_factory.line_graph(2, opts=[{'feerates': (1000,) * 4,
-                                               'min-emergency-msat': 546000},
+                                               'min-emergency-msat': 546000,
+                                               broken_log: 'overgrind: short signature length'},
                                               {'feerates': (1000,) * 4,
                                                'disconnect': ['-WIRE_UPDATE_FAIL_HTLC']}])
     assert 'anchors/even' in only_one(l1.rpc.listpeerchannels()['channels'])['channel_type']['names']
@@ -3835,7 +3836,7 @@ def test_closing_anchorspend_htlc_tx_rbf(node_factory, bitcoind):
     total_weight = sum([d['weight'] for d in details])
     total_fees = sum([float(d['fees']['base']) * 100_000_000 for d in details])
     total_feerate_perkw = total_fees / total_weight * 1000
-    assert 2000 - 1 < total_feerate_perkw < 2000 + 1
+    assert did_short_sig(l1) or 2000 - 1 < total_feerate_perkw < 2000 + 1
 
     # But we don't mine it!  And fees go up again!
     l1.set_feerates((3000, 3000, 3000, 3000))
@@ -3850,7 +3851,7 @@ def test_closing_anchorspend_htlc_tx_rbf(node_factory, bitcoind):
     total_weight = sum([d['weight'] for d in details])
     total_fees = sum([float(d['fees']['base']) * 100_000_000 for d in details])
     total_feerate_perkw = total_fees / total_weight * 1000
-    assert 3000 - 1 < total_feerate_perkw < 3000 + 1
+    assert did_short_sig(l1) or 3000 - 1 < total_feerate_perkw < 3000 + 1
 
     # And now we'll get it in (there's some rounding, so feerate a bit lower!)
     bitcoind.generate_block(1, needfeerate=2990)