For local media imports, copy to tempfile before importing

Apparently `wp_handle_sideload` performs a`rename()` operation on the
file it's sideloading. In this case thats an unexpected side effect -
you don't want the original image to go away when importing it to
WordPress.
Add behat features to test against local files

One test for failure on trying to import non-existant image,
and one to make sure that local images are not removed on import