pytest: add test to demonstrate gossip_store misordering node announcements.

We usually lose the node announcement on restart, because the
node_announcement message is ignored by gossmap, as it doesn't (yet!) know of the node, since the
channel_announcement does not precede the node_announcement.

This is supposed to be detected and fixed by gossipd, but this simple test shows that it is not!

```
FAILED tests/test_gossip.py::test_gossmap_lost_node - AssertionError: assert {'nodes': [{'nodeid': '022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59', 'alias': 'SILENTARTIST-v25.12-2-g703851b', 'color': '022d22', 'last_timestamp': 1765172273, 'features': '8898880a8a59a1', 'addresses': []}, {'nodeid': '0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518', 'alias': 'JUNIORBEAM-v25.12-2-g703851b', 'color': '0266e4', 'last_timestamp': 1765172273, 'features': '8898880a8a59a1', 'addresses': []}, {'nodeid': '035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d'}, {'nodeid': '0382ce59ebf18be7d84677c2e35f23294b9992ceca95491fcf8a56c6cb2d9de199', 'alias': 'JUNIORFELONY-v25.12-2-g703851b', 'color': '0382ce', 'last_timestamp': 1765172273, 'features': '8898880a8a59a1', 'addresses': []}]} == {'nodes': [{'nodeid': '022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59', 'alias': 'SILENTARTIST-v25.12-2-g703851b', 'color': '022d22', 'last_timestamp': 1765172273, 'features': '8898880a8a59a1', 'addresses': []}, {'nodeid': '0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518', 'alias': 'JUNIORBEAM-v25.12-2-g703851b', 'color': '0266e4', 'last_timestamp': 1765172273, 'features': '8898880a8a59a1', 'addresses': []}, {'nodeid': '035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d', 'alias': 'HOPPINGFIRE-v25.12-2-g703851b', 'color': '035d2b', 'last_timestamp': 1765172273, 'features': '8898880a8a59a1', 'addresses': []}, {'nodeid': '0382ce59ebf18be7d84677c2e35f23294b9992ceca95491fcf8a56c6cb2d9de199', 'alias': 'JUNIORFELONY-v25.12-2-g703851b', 'color': '0382ce', 'last_timestamp': 1765172273, 'features': '8898880a8a59a1', 'addresses': []}]}
  
  Differing items:
  {'nodes': [{'addresses': [], 'alias': 'SILENTARTIST-v25.12-2-g703851b', 'color': '022d22', 'features': '8898880a8a59a1...}, {'addresses': [], 'alias': 'JUNIORFELONY-v25.12-2-g703851b', 'color': '0382ce', 'features': '8898880a8a59a1', ...}]} != {'nodes': [{'addresses': [], 'alias': 'SILENTARTIST-v25.12-2-g703851b', 'color': '022d22', 'features': '8898880a8a59a1...}, {'addresses': [], 'alias': 'JUNIORFELONY-v25.12-2-g703851b', 'color': '0382ce', 'features': '8898880a8a59a1', ...}]}
  
  Full diff:
    {
        'nodes': [
            {
                'addresses': [],
                'alias': 'SILENTARTIST-v25.12-2-g703851b',
                'color': '022d22',
                'features': '8898880a8a59a1',
                'last_timestamp': 1765172273,
                'nodeid': '022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59',
            },
            {
                'addresses': [],
                'alias': 'JUNIORBEAM-v25.12-2-g703851b',
                'color': '0266e4',
                'features': '8898880a8a59a1',
                'last_timestamp': 1765172273,
                'nodeid': '0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518',
            },
            {
  -             'addresses': [],
  -             'alias': 'HOPPINGFIRE-v25.12-2-g703851b',
  -             'color': '035d2b',
  -             'features': '8898880a8a59a1',
  -             'last_timestamp': 1765172273,
                'nodeid': '035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d',
            },
            {
                'addresses': [],
                'alias': 'JUNIORFELONY-v25.12-2-g703851b',
                'color': '0382ce',
                'features': '8898880a8a59a1',
                'last_timestamp': 1765172273,
                'nodeid': '0382ce59ebf18be7d84677c2e35f23294b9992ceca95491fcf8a56c6cb2d9de199',
            },
        ],
    }

```

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

Author: rustyrussell

tests/test_gossip.py

@@ -2370,3 +2370,22 @@ def test_incoming_unreasonable(node_factory):
     wait_for(lambda: [c['updates']['remote']['fee_base_msat'] for c in l3.rpc.listpeerchannels()['channels']] == [100000000, 100000000])
     l3.restart()
     l3.rpc.listincoming()
+
+
+@pytest.mark.xfail(strict=True)
+def test_gossmap_lost_node(node_factory, bitcoind):
+    l1, l2, l3, l4 = node_factory.line_graph(4, wait_for_announce=True)
+
+    scid23 = only_one(l2.rpc.listpeerchannels(l3.info['id'])['channels'])['short_channel_id']
+    l2.rpc.close(l3.info['id'])
+    bitcoind.generate_block(13, wait_for_mempool=1)
+    wait_for(lambda: l1.rpc.listchannels(scid23) == {'channels': []})
+
+    pre_channels = l1.rpc.listchannels()
+    pre_nodes = l1.rpc.listnodes()
+    l1.restart()
+    post_channels = l1.rpc.listchannels()
+    post_nodes = l1.rpc.listnodes()
+
+    assert post_channels == pre_channels
+    assert post_nodes == pre_nodes