Tests: ensure temporary directories and files are removed after tests

- QSettings object needs special care, as file was re-created on exit
- Append a cleanup function as soon as possible to ensure temporary dirs & files are removed in any case
- prefer addCleanup() over tearDown(), keeping creation and cleanup together

test/test_file.py

@@ -43,13 +43,11 @@ class TestPreserveTimes(PicardTestCase):
 
     def setUp(self):
         super().setUp()
-        self.tmp_directory = mkdtemp()
+        self.tmp_directory = mkdtemp(suffix=self.__class__.__name__)
+        self.addCleanup(shutil.rmtree, self.tmp_directory)
         filepath = os.path.join(self.tmp_directory, 'a.mp3')
         self.file = File(filepath)
 
-    def tearDown(self):
-        shutil.rmtree(self.tmp_directory)
-
     def _create_testfile(self):
         # create a dummy file
         with open(self.file.filename, 'w') as f: