test: adding 2 additional tests to user.py

Author: didier-durand

tests/auth/test_user.py

@@ -1,9 +1,17 @@
 import unittest
+from inspect import isabstract
 
-from a2a.auth.user import UnauthenticatedUser
+from a2a.auth.user import UnauthenticatedUser, User
 
+class TestUser(unittest.TestCase):
+    def test_is_abstract(self):
+        assert isabstract(User)
 
 class TestUnauthenticatedUser(unittest.TestCase):
+
+    def test_is_user_subclass(self):
+        assert issubclass(UnauthenticatedUser, User)
+
     def test_is_authenticated_returns_false(self):
         user = UnauthenticatedUser()
         self.assertFalse(user.is_authenticated)