Taxonomy: Fix warnings thrown by custom term count callbacks.

Add a `use` to a closure to avoid an undefined variable throwing a warning. Adds unit tests to ensure the custom callbacks run as expected when defined.

Follow up to [49141].
Props ocean90, dd32.
Fixes #40351.


Built from https://develop.svn.wordpress.org/trunk@49171


git-svn-id: http://core.svn.wordpress.org/trunk@48933 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/class-wp-taxonomy.php

@@ -426,7 +426,7 @@ final class WP_Taxonomy {
 			is_callable( $args['update_count_callback'] ) &&
 			empty( $args['update_count_by_callback'] )
 		) {
-			$args['update_count_by_callback'] = function( $tt_ids, $taxonomy, $modify_by ) {
+			$args['update_count_by_callback'] = function( $tt_ids, $taxonomy ) use ( $args ) {
 				return call_user_func( $args['update_count_callback'], $tt_ids, $taxonomy );
 			};
 		}

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.6-alpha-49170';
+$wp_version = '5.6-alpha-49171';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.