(PUP-716) Fix up tests after FileSystem API change (unit/provider)

Still one test failing on expectation
(PUP-761) Fix issues with unit/indirector/key/filespec FileSystem API

Mocking was wrong expected strings not the Patnames being passed.
(PUP-761) Make most FileSystem API change related test pass
(maint) Remove unused file_system/file.rb

The functionality was moved to FileSystem, and to
file_system/file_impl.rb. The now removed file was kept for reference.
Now no longer needed.