Fix: Do not load cakephp-plugins from outside of ROOT

Running tests on a vendor installed cakephp will cause plugins from its
app to be loaded.

src/Core/Plugin.php

@@ -189,7 +189,7 @@ protected static function _loadConfig()
         $vendorFile = dirname(dirname(__DIR__)) . DIRECTORY_SEPARATOR . 'cakephp-plugins.php';
         if (!file_exists($vendorFile)) {
             $vendorFile = dirname(dirname(dirname(dirname(__DIR__)))) . DIRECTORY_SEPARATOR . 'cakephp-plugins.php';
-            if (!file_exists($vendorFile)) {
+            if (strpos(ROOT, $vendorFile) === false || !file_exists($vendorFile)) {
                 Configure::write(['plugins' => []]);
 
                 return;