fix sloppy Server event loop management, add Server tests

tests/test_handler.py

@@ -0,0 +1,7 @@
+# from websockets.sync.client import connect
+# 
+# def hello():
+#     with connect("ws://localhost:8765") as websocket:
+#         websocket.send("Hello world!")
+#         message = websocket.recv()
+#         print(f"Received: {message}")