CRM_Core_I18n - Fix usage of getPath('[foo]/.')

The interpretation of `/.` is evolving per civicrm#16735:

* When this code was first written, it was unspecified/variable whether the value `[foo]/.` would end in `/`
* During most of the testing of 5.23.beta1, this was defined to always return `/`
* During a regression fix in 5.23.1, we're flipping it back the other way so that `[foo]/.` never ends in `/`.

CRM/Core/I18n.php

@@ -282,7 +282,7 @@ public function strarg($str) {
    * @return string
    */
   public static function getResourceDir() {
-    return \Civi::paths()->getPath('[civicrm.l10n]/.');
+    return CRM_Utils_File::addTrailingSlash(\Civi::paths()->getPath('[civicrm.l10n]/.'));
   }
 
   /**