Make file-admin test easier to extend
Make file-admin test easier to extend

Currently the unit tests for file-admin are tied to the local
implementation. However, all other implementations of the file-admin
(e..g S3) should maintain the same invariants as the local one. As such,
this change makes it easier to copy the local file-admin tests for other
storage backends.

flask-admin/flask-admin,flask-admin/flask-admin,flask-admin/flask-admin,flask-admin/flask-admin