Use `run_subtests` utility in FSDP `test_state_dict_save_load_flow` test (#95090)
Converts the single-instance of `self.subTest` in `test_fsdp_state_dict.py` to use the `run_subtests` utility.
Related: #84171
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95090
Approved by: https://github.com/awgu
