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.

.gitignore

@@ -6,6 +6,8 @@ dist/
 docs/mkdocs/site
 test-results.xml
 __pycache__
+# Vim
+.*.sw*
 # generated by setuptools_scm
 bumble/_version.py
 .vscode/launch.json