From the manual page:
-P prefix
--directory-prefix=prefix
Set directory prefix to prefix. The directory prefix is the
directory where all other files and sub-directories will be
saved to, i.e. the top of the retrieval tree. The default
is . (the current directory).
So you need to add -P /tmp/cron_test/
(short form) or --directory-prefix=/tmp/cron_test/
(long form) to your command. Also note that if the directory does not exist it will get created.