tests: clear util.callback_mgr between test cases

util.callback_mgr.callbacks was not getting properly cleared between tests.
Every time an Abstract_Wallet or an LNWorker (or many other subclasses of EventListener) is instantiated,
self.register_callbacks() is called in __init__, which puts callbacks into util.callback_mgr.callbacks.
These are only cleaned up if we explicitly call Abstract_Wallet.stop() or LNWorker.stop() later, which we usually do not do in the tests.

As a result, when running multiple unit tests in a row, lots of objects created in a given testcase are never GC-ed and leak into subsequent tests. This is not only a memory leak, but wastes compute too: when events are triggered and cbs get called, these old objects also have their cbs called.

After running all (~1061) unit tests, I observe util.callback_mgr.callbacks had 30 events with a total of 3156 callbacks stored.

On my laptop, running all unit tests previously took ~115 sec, and now it takes ~73 sec.

tests/__init__.py

@@ -75,6 +75,7 @@ class ElectrumTestCase(unittest.IsolatedAsyncioTestCase, Logger):
         util._asyncio_event_loop = loop
 
     def tearDown(self):
+        util.callback_mgr.clear_all_callbacks()
         shutil.rmtree(self.electrum_path)
         super().tearDown()
         util._asyncio_event_loop = None  # cleared here, at the ~last possible moment. asyncTearDown is too early.

tests/test_lnpeer.py

@@ -1117,12 +1117,8 @@ class TestPeerDirect(TestPeer):
         util.register_callback(on_htlc_fulfilled, ["htlc_fulfilled"])
         util.register_callback(on_htlc_failed, ["htlc_failed"])
 
-        try:
-            with self.assertRaises(SuccessfulTest):
-                await f()
-        finally:
-            util.unregister_callback(on_htlc_fulfilled)
-            util.unregister_callback(on_htlc_failed)
+        with self.assertRaises(SuccessfulTest):
+            await f()
 
     async def test_payment_recv_mpp_confusion2(self):
         """Regression test for https://github.com/spesmilo/electrum/security/advisories/GHSA-8r85-vp7r-hjxf"""
@@ -1191,12 +1187,8 @@ class TestPeerDirect(TestPeer):
         util.register_callback(on_htlc_fulfilled, ["htlc_fulfilled"])
         util.register_callback(on_htlc_failed, ["htlc_failed"])
 
-        try:
-            with self.assertRaises(SuccessfulTest):
-                await f()
-        finally:
-            util.unregister_callback(on_htlc_fulfilled)
-            util.unregister_callback(on_htlc_failed)
+        with self.assertRaises(SuccessfulTest):
+            await f()
 
     async def test_legacy_shutdown_low(self):
         await self._test_shutdown(alice_fee=100, bob_fee=150)