open_tcp_server_transport: allow explicit sock as input.

When a user doesn't need an exact port, but cares more about getting
SOME unused port, they can do:
* Create a socket outside with port=None or port=0.
* Use socket.getsockname()[1] to get the allocated port and pass to the
TCP client somehow.
* Use the created socket to create a TCP server transport.

Use-case: unit-testing embedded software that implements a BLE host. The
controller will be a Bumble controller, connected to the host via a TCP
channel.
* The host will have a TCP-client HCI transport for testing.
* The pytest setup code will allocate the TCP server and pass the port
number to the host.

Also add some unittests with python mock.

bumble/transport/tcp_server.py

@@ -18,6 +18,7 @@
 from __future__ import annotations
 import asyncio
 import logging
+import socket
 
 from .common import Transport, StreamPacketSource
 
@@ -28,6 +29,12 @@ logger = logging.getLogger(__name__)
 
 
 # -----------------------------------------------------------------------------
+
+# A pass-through function to ease mock testing.
+async def _create_server(*args, **kw_args):
+    await asyncio.get_running_loop().create_server(*args, **kw_args)
+
+
 async def open_tcp_server_transport(spec: str) -> Transport:
     '''
     Open a TCP server transport.
@@ -38,7 +45,22 @@ async def open_tcp_server_transport(spec: str) -> Transport:
 
     Example: _:9001
     '''
+    local_host, local_port = spec.split(':')
+    return await _open_tcp_server_transport_impl(
+        host=local_host if local_host != '_' else None, port=int(local_port)
+    )
 
+
+async def open_tcp_server_transport_with_socket(sock: socket.socket) -> Transport:
+    '''
+    Open a TCP server transport with an existing socket.
+
+    One reason to use this variant is to let python pick an unused port.
+    '''
+    return await _open_tcp_server_transport_impl(sock=sock)
+
+
+async def _open_tcp_server_transport_impl(**kwargs) -> Transport:
     class TcpServerTransport(Transport):
         async def close(self):
             await super().close()
@@ -77,13 +99,10 @@ async def open_tcp_server_transport(spec: str) -> Transport:
             else:
                 logger.debug('no client, dropping packet')
 
-    local_host, local_port = spec.split(':')
     packet_source = StreamPacketSource()
     packet_sink = TcpServerPacketSink()
-    await asyncio.get_running_loop().create_server(
-        lambda: TcpServerProtocol(packet_source, packet_sink),
-        host=local_host if local_host != '_' else None,
-        port=int(local_port),
+    await _create_server(
+        lambda: TcpServerProtocol(packet_source, packet_sink), **kwargs
     )
 
     return TcpServerTransport(packet_source, packet_sink)