Remove any existing IPC hanlders/listeners before assigning new ones inside the downloadsManager (#2493)

* Remove any existing IPC hanlders/listeners before assigning new ones inside the downloadsManager

* Mock more ipx functions during tests

src/main/menus/app.test.js

@@ -32,6 +32,8 @@ jest.mock('electron', () => {
             emit: jest.fn(),
             handle: jest.fn(),
             on: jest.fn(),
+            removeHandler: jest.fn(),
+            removeListener: jest.fn(),
         },
         Notification: NotificationMock,
     };