[torch_checkpointing][test] use real barrier for test

[aelavender]

This test does CheckpointWriterConfig(barrier_config=None), which I'm about to disallow without further config.

• In meta-pytorch/torch_checkpointing@32f50c1, we change the default barrier from None to DefaultStoreBarrier. This is does not affect Titan; it currently explicitly sets up a TCPStoreBarrier.
• Current torch_checkpointing behaviour is to write to a temp dir and atomically rename iff a barrier is configured. I am soon to make this an explicit flag (defaulting to True), which will make this test break. So fixing the test before I make the api change.

Tested with pytest ./tests/unit_tests/cpu/test_torch_checkpointing.py