mv: no-op when paths are the same

[DavidKatz-il]

fixes: fsspec/s3fs#711

Previous: If you run mv('path', 'path'), the mv function will call cp and then rm, resulting in the removal of the original 'path'.
Revised: If you run mv('path', 'path'), the mv function will not perform any action as the source and destination paths are the same.

[martindurant]

Any chance of adding a test in one of the backends?

[DavidKatz-il]

Where should we add this test (which test file)?

[martindurant]

I suggest fsspec.implementations.tests.test_memory and use a test fixture m (an instance of MemoryFileSystem) like all the other tests there.