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

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.

Testing Done:
Unit tests pass.

Made use of the new helpers in an upcoming change.

Reviewed at https://reviews.reviewboard.org/r/10707/

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