fix(core): do not redefine TEMPEST_START (#806)

Author: innocenzi

src/Tempest/Core/src/Kernel.php

@@ -39,7 +39,9 @@ public static function boot(
         array $discoveryLocations = [],
         ?Container $container = null,
     ): self {
-        define('TEMPEST_START', value: hrtime(true));
+        if (! defined('TEMPEST_START')) {
+            define('TEMPEST_START', value: hrtime(true));
+        }
 
         return (new self(
             root: $root,