tests: fix TestLNTransport probabilistic failure

lucky CI run: https://github.com/spesmilo/electrum/runs/9273471816

electrum/tests/test_lntransport.py

@@ -77,8 +77,8 @@ class TestLNTransport(ElectrumTestCase):
                 await group.spawn(read_messages(t, messages_sent_by_client))
                 await group.spawn(write_messages(t, messages_sent_by_server))
             responder_shaked.set()
-        async def connect():
-            peer_addr = LNPeerAddr('127.0.0.1', 42898, responder_key.get_public_key_bytes())
+        async def connect(port: int):
+            peer_addr = LNPeerAddr('127.0.0.1', port, responder_key.get_public_key_bytes())
             t = LNTransport(initiator_key.get_secret_bytes(), peer_addr, proxy=None)
             await t.handshake()
             async with OldTaskGroup() as group:
@@ -87,10 +87,11 @@ class TestLNTransport(ElectrumTestCase):
             server_shaked.set()
 
         async def f():
-            server = await asyncio.start_server(cb, '127.0.0.1', 42898)
+            server = await asyncio.start_server(cb, '127.0.0.1', port=None)
+            server_port = server.sockets[0].getsockname()[1]
             try:
                 async with OldTaskGroup() as group:
-                    await group.spawn(connect())
+                    await group.spawn(connect(port=server_port))
                     await group.spawn(responder_shaked.wait())
                     await group.spawn(server_shaked.wait())
             finally: