tests: add basic tests for util.CallbackManager and EventListener

diff --git a/tests/test_callbackmgr.py b/tests/test_callbackmgr.py
new file mode 100644
index 0000000..1a390e7
--- /dev/null
+++ b/tests/test_callbackmgr.py
@@ -0,0 +1,101 @@
+import asyncio
+
+from electrum import util
+from electrum.util import EventListener, event_listener, trigger_callback
+from electrum.utils.memory_leak import count_objects_in_memory
+
+from . import ElectrumTestCase
+
+
+class MyEventListener(EventListener):
+    def __init__(self):
+        self._satoshi_cnt = 0
+        self._hal_cnt = 0
+
+    def start(self):
+        self.register_callbacks()
+
+    def stop(self):
+        self.unregister_callbacks()
+
+    @event_listener
+    async def on_event_satoshi_moves_his_coins(self, arg1, arg2):
+        self._satoshi_cnt += 1
+
+    @event_listener
+    def on_event_hal_moves_his_coins(self, arg1, arg2):  # non-async
+        self._hal_cnt += 1
+
+
+def _count_all_callbacks() -> int:
+    return sum(len(cbs) for cbs in util.callback_mgr.callbacks.values())
+
+
+async def fast_sleep():
+    # sleep a few event loop iterations
+    for i in range(5):
+        await asyncio.sleep(0)
+
+
+class TestCallbackMgr(ElectrumTestCase):
+
+    def test_multiple_calls_to_register_callbacks(self):
+        self.assertEqual(0, _count_all_callbacks())
+        el1 = MyEventListener()
+        el2 = MyEventListener()
+        self.assertEqual(0, _count_all_callbacks())
+        el1.start()
+        self.assertEqual(2, _count_all_callbacks())
+        el2.start()
+        self.assertEqual(4, _count_all_callbacks())
+        el1.start()
+        self.assertEqual(6, _count_all_callbacks())
+        el1.stop()
+        self.assertEqual(4, _count_all_callbacks())
+        el1.stop()
+        self.assertEqual(2, _count_all_callbacks())
+        el1.stop()
+        self.assertEqual(2, _count_all_callbacks())
+        el2.stop()
+        self.assertEqual(0, _count_all_callbacks())
+
+    async def test_trigger_callback(self):
+        el1 = MyEventListener()
+        el1.start()
+        el2 = MyEventListener()
+        el2.start()
+        # trigger some cbs
+        self.assertEqual(el1._satoshi_cnt, 0)
+        self.assertEqual(el1._hal_cnt, 0)
+        trigger_callback('satoshi_moves_his_coins', 0, 0)
+        trigger_callback('satoshi_moves_his_coins', 0, 0)
+        trigger_callback('satoshi_moves_his_coins', 0, 0)
+        trigger_callback('hal_moves_his_coins', 0, 0)
+        await fast_sleep()
+        self.assertEqual(el1._satoshi_cnt, 3)
+        self.assertEqual(el2._satoshi_cnt, 3)
+        self.assertEqual(el1._hal_cnt, 1)
+        self.assertEqual(el2._hal_cnt, 1)
+        # stop one listener, see new triggers are only seen by other one still running
+        el1.stop()
+        trigger_callback('satoshi_moves_his_coins', 0, 0)
+        trigger_callback('hal_moves_his_coins', 0, 0)
+        await fast_sleep()
+        self.assertEqual(el1._satoshi_cnt, 3)
+        self.assertEqual(el2._satoshi_cnt, 4)
+        self.assertEqual(el1._hal_cnt, 1)
+        self.assertEqual(el2._hal_cnt, 2)
+
+    async def test_gc(self):
+        objmap = count_objects_in_memory([MyEventListener])
+        self.assertEqual(len(objmap[MyEventListener]), 0)
+        el1 = MyEventListener()
+        el1.start()
+        el2 = MyEventListener()
+        el2.start()
+        objmap = count_objects_in_memory([MyEventListener])
+        self.assertEqual(len(objmap[MyEventListener]), 2)
+        el1.stop()
+        del el1
+        objmap = count_objects_in_memory([MyEventListener])
+        self.assertEqual(len(objmap[MyEventListener]), 1)