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.


git-svn-id: https://develop.svn.wordpress.org/trunk@44785 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/includes/abstract-testcase.php

@@ -1062,7 +1062,7 @@ abstract class WP_UnitTestCase_Base extends PHPUnit_Framework_TestCase {
 		}
 
 		$attachment = array(
-			'post_title'     => basename( $upload['file'] ),
+			'post_title'     => wp_basename( $upload['file'] ),
 			'post_content'   => '',
 			'post_type'      => 'attachment',
 			'post_parent'    => $parent_post_id,