Build/Test: Fix incorrect hook being used to separate Server-Timing metrics in performance tests.

The `wp-before-template` and `wp-template` metric intend to separate WordPress core's own bootstrap process from any logic that is part of the eventual template loaded. The appropriate hook for that is the `template_include` filter as that occurs right before the template file is included.

Props mukesh27, joemcgill.
Fixes #58674.


git-svn-id: https://develop.svn.wordpress.org/trunk@56162 602fd350-edb4-49c9-b593-d223f7449a82

tests/performance/wp-content/mu-plugins/server-timing.php

@@ -1,8 +1,8 @@
 <?php
 
-add_action(
-	'template_redirect',
-	static function() {
+add_filter(
+	'template_include',
+	static function( $template ) {
 
 		global $timestart;
 
@@ -38,6 +38,8 @@ add_action(
 			},
 			PHP_INT_MIN
 		);
+
+		return $template;
 	},
 	PHP_INT_MAX
 );