Test cases do not schedule to GUI thread as expected

* Test cases do not use typical `start_management`, instead creating MainWindow directly https://github.com/angr/angr-management/blob/5872bcd0f5487b22590124ea4e8c459bc26af86f/tests/test_workflow.py#L16
* `GlobalInfo.gui_thread` is never set (by `start_management`), so `is_gui_thread` always returns `True` https://github.com/angr/angr-management/blob/5872bcd0f5487b22590124ea4e8c459bc26af86f/angrmanagement/logic/threads.py#L126
* Because `is_gui_thread` always returns `True`, any callable passed to `gui_thread_schedule[_async]` will actually be run on caller thread
* I had an issue with deadlocking after some Qt objects were created with parents on worker threads because of this. Ideally the test cases would also run through the `start_management` init path.