message: added function 'rm_file()' and test

tests/test_message.py

@@ -874,3 +874,22 @@ This is a question.
 {Answer.txt_header}
 This is an answer."""
         self.assertEqual(self.message.to_str(with_tags=True, with_file=True), expected_output)
+
+
+class MessageRmFileTestCase(unittest.TestCase):
+    def setUp(self) -> None:
+        self.file = tempfile.NamedTemporaryFile(delete=False, suffix=msg_suffix)
+        self.file_path = pathlib.Path(self.file.name)
+        self.message = Message(Question('This is a question.'),
+                               file_path=self.file_path)
+        self.message.to_file()
+
+    def tearDown(self) -> None:
+        self.file.close()
+        self.file_path.unlink(missing_ok=True)
+
+    def test_rm_file(self) -> None:
+        assert self.message.file_path
+        self.assertTrue(self.message.file_path.exists())
+        self.message.rm_file()
+        self.assertFalse(self.message.file_path.exists())