WP_Query: Add tests for the combination of orderby=post__in and `or…

…der`.

This commit adds test cases for the interaction (or more accurately, lack of
interaction) between `orderby=post__in` and the `order` parameter.

Props fibonaccina.
See #39055.

Built from https://develop.svn.wordpress.org/trunk@40056


git-svn-id: http://core.svn.wordpress.org/trunk@39993 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-40055';
+$wp_version = '4.8-alpha-40056';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.