Add and fix unittests for NAS

[graykode]

I compared both the list_object function of minio and the list_object function that I directly override(NAS class). At this time, only when the prefix is'metadata/test/', there is a problem that the two do not match. Be careful about this.

Add unittests for NAS

• data saver and data loader
• model saver and model loader
• optimizer saver and optimizer loader

As one problem, we found a problem that unittest.skip cannot be called in the same class. Therefore, this stackoverflow answer solved this.