Added test for ValidationError.messages

tests/test_exceptions/test_validation_error.py

+import unittest
+
+from django.core.exceptions import ValidationError
+
+
+class TestValidationError(unittest.TestCase):
+    def test_messages_concatenates_error_dict_values(self):
+        message_dict = {}
+        with self.assertRaises(TypeError):
+            ValidationError(message_dict).messages
+        message_dict['field1'] = ['E1', 'E2']
+        exception = ValidationError(message_dict)
+        self.assertEqual(sorted(exception.messages), ['E1', 'E2'])
+        message_dict['field2'] = ['E3', 'E4']
+        exception = ValidationError(message_dict)
+        self.assertEqual(sorted(exception.messages), ['E1', 'E2', 'E3', 'E4'])