tests: Add a Pytest fixture for Updater

Add a Pytest fixture that returns an UpdaterThread instance which has
its own unique settings directory. Note that the UpdaterThread instance
needs to be slightly nerfed, so that it doesn't rely on Qt functionality
or any isolation providers.

tests/gui/__init__.py

@@ -0,0 +1,40 @@
+from pathlib import Path
+from typing import Optional
+from unittest import mock
+
+import pytest
+from pytest import MonkeyPatch
+from pytest_mock import MockerFixture
+
+from dangerzone import util
+from dangerzone.gui import Application
+from dangerzone.gui.logic import DangerzoneGui
+from dangerzone.gui.updater import UpdaterThread
+from dangerzone.isolation_provider.dummy import Dummy
+
+
+def generate_isolated_updater(
+    tmp_path: Path,
+    monkeypatch: MonkeyPatch,
+    app_mocker: MockerFixture,
+) -> UpdaterThread:
+    """Generate an Updater class with its own settings."""
+    app = app_mocker.MagicMock()
+
+    dummy = Dummy()
+    # XXX: We can monkey-patch global state without wrapping it in a context manager, or
+    # worrying that it will leak between tests, for two reasons:
+    #
+    # 1. Parallel tests in PyTest take place in different processes.
+    # 2. The monkeypatch fixture tears down the monkey-patch after each test ends.
+    monkeypatch.setattr(util, "get_config_dir", lambda: tmp_path)
+    dangerzone = DangerzoneGui(app, isolation_provider=dummy)
+    updater = UpdaterThread(dangerzone)
+    return updater
+
+
+@pytest.fixture
+def updater(
+    tmp_path: Path, monkeypatch: MonkeyPatch, mocker: MockerFixture
+) -> UpdaterThread:
+    return generate_isolated_updater(tmp_path, monkeypatch, mocker)