Use temporary variable for array in `Tests_Customize_Setting::test_preview_standard_types_multidimensional()`.

The syntax previously used - `call_user_func( 'foo' )['bar']`, where `foo()`
returns an array - is not valid on all supported versions of PHP, and was
breaking the CI builds.

See #30988.
Built from https://develop.svn.wordpress.org/trunk@31339


git-svn-id: http://core.svn.wordpress.org/trunk@31320 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.2-alpha-31338';
+$wp_version = '4.2-alpha-31339';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.