tests/lib/libc/stdio/t_printf: Add another %La test.

This one was adapted from the screw case shown in

https://mail-index.netbsd.org/tech-userlevel/2020/04/11/msg012329.html

which wasn't broken in our libc, but which nevertheless prompted us
to commit a wrong and apparently untested patch that has rendered
printf %La broken for the last four years, which is a little
embarrassing.  (The part of that patch that led to a buffer overrun
has been worked around, so now the output is just incorrect.)

PR lib/56937: printf(3) long double %a formatting is broken

(riastradh)