rename ckpt test (pytorch#825)

## Description

Rename the previous ckpt index file test to ckpt test to reflect its
covering of both index file and params saving.

- [x] Has code been commented, particularly in hard-to-understand areas?
- [x] Have you made corresponding changes to the documentation?

Co-authored-by: Eddy <ogolaedd@meta.com>

test/local_test_ckpt_index_file.py → test/local_test_checkpoint.py

@@ -193,7 +193,7 @@ def main(args: List[str | int] = None) -> None:
     main()
 
 
-class LocalIndexMetadataTest(unittest.TestCase):
+class LocalCheckpointTest(unittest.TestCase):
     def test_index_file(self):
         import random