Make test case more resilient

When ran from a root directory the test case failed, because the open_basedir
restriction for "../[…]" won't kick in. Therefore we change the current
working directory to the test case's directory, as discussed on internals,
see <http://news.php.net/php.internals/95585>.

ext/sqlite3/tests/sqlite3_21_security.phpt

@@ -6,6 +6,8 @@ SQLite3 open_basedir checks
 open_basedir=.
 --FILE--
 <?php
+chdir(__DIR__);
+
 $directory = dirname(__FILE__) . '/';
 $file = uniqid() . '.db';