don't manually assign port during test

fixes #89
instead of saving the multiaddr received in the listen method of the
tcp transport class, read what the service is actually listening on
this allow to use 0 as port and let the OS choose a free port for us

tests/transport/test_tcp.py

@@ -0,0 +1,20 @@
+import asyncio
+
+import pytest
+
+from transport.tcp.tcp import _multiaddr_from_socket
+
+
+@pytest.mark.asyncio
+async def test_multiaddr_from_socket():
+    def handler(r, w):
+        pass
+
+    server = await asyncio.start_server(handler, '127.0.0.1', 8000)
+    assert str(_multiaddr_from_socket(server.sockets[0])) == '/ip4/127.0.0.1/tcp/8000'
+
+    server = await asyncio.start_server(handler, '127.0.0.1', 0)
+    addr = _multiaddr_from_socket(server.sockets[0])
+    assert addr.value_for_protocol('ip4') == '127.0.0.1'
+    port = addr.value_for_protocol('tcp')
+    assert int(port) > 0