This enhances the create_file_attachment() unit test helper, and

related helpers, to make it easier to create file attachments associated

with a FileAttachmentHistory, and to provide data to populate the

attachment with.
It also introduces create_file_attachment_history(), which unit tests

can use to create that FileAttachmentHistory. It will automatically

set its display position on a review request.
In an effort to help keep things maintainable going forward,

create_file_attachment() and create_user_file_attachment() have been

simplified to take fewer arguments and point to the internal helper,

wihch has been made public for documentation purposes.

Unit tests pass.
Made use of the new helpers in an upcoming change.