tests: use non-empty string for static salt

As pointed out by @klap-in, an empty string may evaluate to false in
some circumstances. This is something we may not want. Using a string
like 'test' should therefore be more robust.

inc/auth.php

@@ -309,7 +309,7 @@ function auth_browseruid() {
  */
 function auth_cookiesalt($addsession = false, $secure = false) {
     if (defined('SIMPLE_TEST')) {
-        return '';
+        return 'test';
     }
     global $conf;
     $file = $conf['metadir'].'/_htcookiesalt';