selftests/user_events: Enable the event before write_fault test in ftrace self-test

The user_event has not be enabled in write_fault test in ftrace
self-test, Just enable it.

Link: https://lkml.kernel.org/r/20230626111344.19136-3-sunliming@kylinos.cn

Acked-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: sunliming <sunliming@kylinos.cn>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>