Fix GH-14368: Test failure in ext/session/tests/gh13856.phpt (#14378)

If the runner overrides session.save_path, the test fails.
Manually set it to a value known to trigger the issue.

ext/session/tests/gh13856.phpt

@@ -6,6 +6,7 @@ session
 session.save_handler=files
 open_basedir=.
 error_reporting=E_ALL
+session.save_path=
 --FILE--
 <?php
 session_set_save_handler(new \SessionHandler(), true);