Set session.gc_probability=0 in bug78624.phpt

We only want to test manually triggered session GC.
Avoid spurious output due to automatic GC.

Author: nikic

ext/session/tests/bug78624.phpt

@@ -3,6 +3,7 @@ Test session_set_save_handler() : session_gc() returns the number of deleted rec
 --INI--
 session.name=PHPSESSID
 session.save_handler=files
+session.gc_probability=0
 --SKIPIF--
 <?php include('skipif.inc'); ?>
 --FILE--