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.

tests/transport_tcp_server_test.py

@@ -0,0 +1,64 @@
+# Copyright 2024 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import asyncio
+import os
+import pytest
+import socket
+import unittest
+from unittest.mock import ANY, patch
+
+from bumble.transport.tcp_server import (
+    open_tcp_server_transport,
+    open_tcp_server_transport_with_socket,
+)
+
+
+class OpenTcpServerTransportTests(unittest.TestCase):
+    def setUp(self):
+        self.patcher = patch('bumble.transport.tcp_server._create_server')
+        self.mock_create_server = self.patcher.start()
+
+    def tearDown(self):
+        self.patcher.stop()
+
+    def test_open_with_spec(self):
+        asyncio.run(open_tcp_server_transport('localhost:32100'))
+        self.mock_create_server.assert_awaited_once_with(
+            ANY, host='localhost', port=32100
+        )
+
+    def test_open_with_port_only_spec(self):
+        asyncio.run(open_tcp_server_transport('_:32100'))
+        self.mock_create_server.assert_awaited_once_with(ANY, host=None, port=32100)
+
+    def test_open_with_socket(self):
+        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+            asyncio.run(open_tcp_server_transport_with_socket(sock=sock))
+        self.mock_create_server.assert_awaited_once_with(ANY, sock=sock)
+
+
+@pytest.mark.skipif(
+    not os.environ.get('PYTEST_NOSKIP', 0),
+    reason='''\
+Not hermetic. Should only run manually with
+  $ PYTEST_NOSKIP=1 pytest tests
+''',
+)
+def test_open_with_real_socket():
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+        sock.bind(('localhost', 0))
+        port = sock.getsockname()[1]
+        assert port != 0
+        asyncio.run(open_tcp_server_transport_with_socket(sock=sock))