[feature] GitHub CRUD tests

I filled in the test stubs for github_update_file, github_delete_file, and github_view_file.

This replaces [PR 1243](https://github.com/CenterForOpenScience/osf/pull/1243) in the old repo, which I am going to close now.

This closes [PR 1278](https://github.com/CenterForOpenScience/osf.io/pull/1278).
Not sure that this should be a redirect. If I were to guess, you are getting redirect to the login page because no `auth` was passed to the original `get` (you can verify this by calling `res.showbrowser()`). Keep in mind that `self.project` is private, so you need auth to be able to the file.