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.
git-svn-id: https://develop.svn.wordpress.org/trunk@38582 602fd350-edb4-49c9-b593-d223f7449a82