pytest: fix test_xpay flake.

```
>       assert len(layers['layers']) == 1
E       AssertionError: assert 2 == 1
E        +  where 2 = len([{'layer': 'xpay', 'persistent': True, 'disabled_nodes': [], 'created_channels': [], 'channel_updates': [], 'constraints': [{'short_channel_id_dir': '45210x2134x44171/0', 'timestamp': 1770341134, 'minimum_msat': 289153519}, {'short_channel_id_dir': '1895x7x1895/1', 'timestamp': 1770341134, 'minimum_msat': 289007015}, {'short_channel_id_dir': '1906x1039x1906/1', 'timestamp': 1770341134, 'minimum_msat': 289008304}, {'short_channel_id_dir': '10070x60x10063/1', 'timestamp': 1770341134, 'minimum_msat': 289005726}, {'short_channel_id_dir': '18772x60x18743/0', 'timestamp': 1770341134, 'minimum_msat': 289005726}, {'short_channel_id_dir': '18623x208x18594/0', 'timestamp': 1770341134, 'minimum_msat': 289004859}, {'short_channel_id_dir': '33935x826x33727/1', 'timestamp': 1770341134, 'maximum_msat': 491501488}], 'biases': [], 'node_biases': []}, {'layer': 'xpay-94', 'persistent': False, 'disabled_nodes': [], 'created_channels': [], 'channel_updates': [], 'constraints': [], 'biases': [], 'node_biases': []}])
```

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

Author: rustyrussell

tests/test_xpay.py

@@ -281,12 +281,12 @@ def test_xpay_fake_channeld(node_factory, bitcoind, chainparams, slow_mode):
     # Should be no reservations left (clean up happens after return though)
     wait_for(lambda: l1.rpc.askrene_listreservations() == {'reservations': []})
 
+    # Wait for temporary layers to be gone too.
+    wait_for(lambda: len(l1.rpc.askrene_listlayers()['layers']) == 1)
+
     # It should remember the information it learned across restarts!
     # FIXME: channeld_fakenet doesn't restart properly, so just redo xpay.
     layers = l1.rpc.askrene_listlayers()
-    # Temporary layers should be gone.
-    assert len(layers['layers']) == 1
-
     l1.rpc.plugin_stop("cln-askrene")
     l1.rpc.plugin_start(os.path.join(os.getcwd(), 'plugins/cln-askrene'))
     layers_after = l1.rpc.askrene_listlayers()