Set the default parent id to 0, instead of -1, in `Tests_Post_Attachments::_make_attachment()`. Prevents `Out of range value for column 'post_parent'` database error without papering over it in core.

See #25282.



git-svn-id: https://develop.svn.wordpress.org/trunk@26004 602fd350-edb4-49c9-b593-d223f7449a82