Tests: Add unit tests for attachment’s file size being included in metadata.
These tests ensure that wp_generate_attachment_metadata() stores the file size of all newly uploaded attachments under the filesize array key. The tests were initially supposed to be committed with [52837].
Follow-up to [52837], [52932], [54861].
Props spacedmonkey, johnwatkins0, mitogh, adamsilverstein, pbearne, SergeyBiryukov.
Fixes #57171.