Add unit test for GTK define

tests/briar/gtk/test_define.py

+# Copyright (c) 2019 Nico Alt
+# SPDX-License-Identifier: AGPL-3.0-only
+# License-Filename: LICENSE.md
+
+from unittest import TestCase
+
+from gi.repository import Gio
+
+from briar.gtk.define import App
+
+
+class TestDefine(TestCase):
+
+    def test_app(self):
+        assert App == Gio.Application.get_default