tests: fix warning in test_lntransport.py::TestLNTransport::test_loop

$ python3 -X dev -m pytest tests/test_lntransport.py

```
=============================== warnings summary ===============================
tests/test_lntransport.py::TestLNTransport::test_loop
  /usr/local/lib/python3.11/asyncio/streams.py:414: ResourceWarning: unclosed <StreamWriter transport=<_SelectorSocketTransport closing fd=15 read=idle write=<idle, bufsize=0>> reader=<StreamReader transport=<_SelectorSocketTransport closing fd=15 read=idle write=<idle, bufsize=0>>>>
    warnings.warn(f"unclosed {self!r}", ResourceWarning)
```

Author: SomberNight

tests/test_lntransport.py

@@ -1,8 +1,10 @@
 import asyncio
+from typing import List
 
 import electrum_ecc as ecc
 
 from electrum import util
+from electrum import lntransport
 from electrum.lntransport import LNPeerAddr, LNResponderTransport, LNTransport, extract_nodeid, split_host_port, ConnStringFormatError
 from electrum.util import OldTaskGroup
 
@@ -71,6 +73,7 @@ async def write_messages(transport, expected_messages):
 
         async def cb(reader, writer):
             t = LNResponderTransport(responder_key.get_secret_bytes(), reader, writer)
+            transports.append(t)
             self.assertEqual(await t.handshake(), initiator_key.get_public_key_bytes())
             async with OldTaskGroup() as group:
                 await group.spawn(read_messages(t, messages_sent_by_client))
@@ -79,12 +82,14 @@ async def cb(reader, writer):
         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, e_proxy=None)
+            transports.append(t)
             await t.handshake()
             async with OldTaskGroup() as group:
                 await group.spawn(read_messages(t, messages_sent_by_server))
                 await group.spawn(write_messages(t, messages_sent_by_client))
             server_shaked.set()
 
+        transports = []  # type: List[lntransport.LNTransportBase]
         async def f():
             server = await asyncio.start_server(cb, '127.0.0.1', port=None)
             server_port = server.sockets[0].getsockname()[1]
@@ -94,6 +99,8 @@ async def f():
                     await group.spawn(responder_shaked.wait())
                     await group.spawn(server_shaked.wait())
             finally:
+                for t in transports:
+                    t.close()
                 server.close()
 
         await f()