File refactor unit test fix

This gets all unit and functional tests passing. I found that the failing tests for remote_file were all testing behavior that has been extracted to implementation classes, which all have passing tests. During this review, I found that tests for gzip disable behavior were not making strong enough assertions about the arguments passed to `Chef::REST.new` to actually verify the correct behavior, so I fixed that and then further refactored those tests to use `let` bindings for greater reusability.