tests/fmt_printf: spice up test
tests/fmt_printf: spice up test

Also include interaction with stdio, as corrupting output on mixing
stdio and fmt is too much of a footgun.

miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,miri64/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,jasonatran/RIOT,kaspar030/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kYc0o/RIOT,RIOT-OS/RIOT