Replace usages of basename() with wp_basename() in order to support multibyte filenames

This is focused on the pieces of code that touch media files and the tests that support them. `basename` isn't multibyte compatible out of the box. See http://php.net/basename and https://bugs.php.net/bug.php?id=62119.

See #43170.
Props Viper007Bond.

Built from https://develop.svn.wordpress.org/trunk@44785


git-svn-id: http://core.svn.wordpress.org/trunk@44617 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/import.php

@@ -99,7 +99,7 @@ function wp_import_handle_upload() {
 
 	// Construct the object array
 	$object = array(
-		'post_title'     => basename( $upload['file'] ),
+		'post_title'     => wp_basename( $upload['file'] ),
 		'post_content'   => $upload['url'],
 		'post_mime_type' => $upload['type'],
 		'guid'           => $upload['url'],