Increases UbuntuCleanerWindow coverage to 100%

The idea is to test UI simulating clicks e.g. `clicked()` and other
events while mocking blocking calls like `run()` or `show()`.

tests/test_app.py

@@ -1,14 +1,29 @@
+import mock
 import unittest
+
 from ubuntucleaner.main import UbuntuCleanerWindow
 
 
-class TestApp(unittest.TestCase):
+class TestUbuntuCleanerWindow(unittest.TestCase):
     def setUp(self):
-        self.window = UbuntuCleanerWindow()
+        with mock.patch("ubuntucleaner.main.Gtk.Window.show"):
+            self.window = UbuntuCleanerWindow()
+
+    def tearDown(self):
+        del self.window
 
     def test_app(self):
         self.assertEqual(self.window.feature_dict, {'janitor': 0})
         self.assertIsNotNone(self.window.aboutdialog)
 
-    def tearDown(self):
-        del self.window
+    def test_on_about_button_clicked(self):
+        with mock.patch("ubuntucleaner.main.Gtk.AboutDialog.run") as m_run:
+            self.window.about_button.clicked()
+        self.assertEqual(m_run.call_args_list, [mock.call()])
+
+    def test_on_mainwindow_destroy(self):
+        with mock.patch("ubuntucleaner.main.Gtk.main_quit") as m_main_quit, \
+                mock.patch("ubuntucleaner.main.exit") as m_exit:
+            self.window.mainwindow.destroy()
+        self.assertEqual(m_main_quit.call_args_list, [mock.call()])
+        self.assertEqual(m_exit.call_args_list, [mock.call()])