xmlrpctests: remove harcoded expiration date from test_user_plugin
    
    The test test_user_plugin is using a hardcoded date for
    password expiration and started failed since we passed this date.
    Replace the hardcoded date with now + 1 year.
    
    Fixes: https://pagure.io/freeipa/issue/8616
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Stanislav Levin <slev@altlinux.org>