Upload tests

Here are some tests for the S3 module.  Production code hasn't changed, but TDD'd it back in.

I think the S3 stuff needs the same open-closed refactor as that happened to the database.
I had to run a full test because I don't know how to get a unit test into this:

https://github.com/kilt-cms/kilt-cms/blob/3a7993bc2cea2d98965db993296b6d1329774519/lib/kilt/upload.rb#L26