Add new testing helpers for working with file attachments and history.

Review Request #10707 — Created Sept. 10, 2019 and submitted — Latest diff uploaded

Review Board

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.