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.
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