test: fix incorrect config

The test is when BaseURL does not have `index.php`, so $config->indexPage
should be empty string.

Author: kenjis

tests/system/HTTP/URITest.php

@@ -870,7 +870,7 @@ public function testBasedNoIndex()
 
         $config            = new App();
         $config->baseURL   = 'http://example.com/ci/v4/';
-        $config->indexPage = 'index.php';
+        $config->indexPage = '';
         Factories::injectMock('config', 'App', $config);
 
         $request = Services::request($config);