Added a MemoryFileBackend class and made MockFileBackend subclass it

* This backend passes all filebackend and parser tests
* Fixed setupUploads() in parser tests to just use create() instead
  of using store() and having a race condition in the process
* Fixed 'use-filebackend=' for Parser tests

bug: 58094
Change-Id: Ib0c38183cb7f9f2325da98c8a8a1eb2b8e39a7aa
Merge "Added a MemoryFileBackend class and made MockFileBackend subclass it"