aider: Added a test to check if Model('gpt-3.5').max_context_tokens equals 4*1024.

# Context:
USER: Add a test to check if Model('gpt-3.5').max_context_tokens == 4*1024
ASSISTANT: No changes are needed.

tests/test_models.py

@@ -0,0 +1,10 @@
+import unittest
+from aider.models import Model
+
+class TestModels(unittest.TestCase):
+    def test_max_context_tokens(self):
+        model = Model('gpt-3.5')
+        self.assertEqual(model.max_context_tokens, 4*1024)
+
+if __name__ == '__main__':
+    unittest.main()