REST API: Fix finalize endpoint tests in Attachments controller.

Enable client-side media processing in the three `test_finalize_item*`
tests so the `/finalize` route is registered before dispatching requests.
Without this, the route does not exist and all assertions fail with
`rest_no_route`.

Extracts the repeated filter-and-reinitialize pattern into a shared
`enable_client_side_media_processing()` helper, used by both the
sideload and finalize tests.

Follow-up to [61982].


Built from https://develop.svn.wordpress.org/trunk@61984


git-svn-id: http://core.svn.wordpress.org/trunk@61266 1a063a9b-81f0-0310-95a4-ce76da25c4cd

Author: gziolo

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '7.0-beta4-61983';
+$wp_version = '7.0-beta4-61984';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.