In `_wp_handle_upload()`, if `test_upload` is set to `false` in the array of dangerous overrides that the function allows, the only thing that happens when an upload fails is more potential breakage.

`$test_uploaded_file` lets is know if `$file['tmp_name']` exists, which allows to exit with an error, instead of continuing to attempt to move the file.

`$test_upload` override is now a noop.

Fixes #28208.


git-svn-id: https://develop.svn.wordpress.org/trunk@30076 602fd350-edb4-49c9-b593-d223f7449a82