Tests: Use `add_filter()` when it's available.

The `tests_add_filter()` helper function directly manipulates the `$wp_filter` global, instead of using `add_filter()`. We can use `add_filter()` when it's available, and fall back to manipulating `$wp_filter` when it isn't, relying on the `$wp_filter` bootstrap code at the top of `plugin.php` to handle conversion.

Props boonebgorges, dd32 and pento: WordPress Thought Leadership Triumvirate.
Fixes #17817.


Built from https://develop.svn.wordpress.org/trunk@38582


git-svn-id: http://core.svn.wordpress.org/trunk@38525 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.7-alpha-38580';
+$wp_version = '4.7-alpha-38582';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.