Fix sfCacheSessionStorageTest with PHP>=7.2

From PHP 7.2 onwards, session functions are more strict and may not
work if output/headers have already been sent out. Using output
buffering prevents this issue.

Author: mentalstring

test/unit/storage/sfCacheSessionStorageTest.php

@@ -12,6 +12,8 @@
 
 require_once __DIR__.'/../../bootstrap/functional.php';
 
+ob_start();
+
 $_test_dir = realpath(__DIR__.'/../../');
 
 require_once $_test_dir.'/../lib/vendor/lime/lime.php';