selftest/cases/glibc.py: fix the override syntax

Fix the override so we actually pass the correct value to glibc.

Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 38fd2120)
Signed-off-by: Steve Sakoman <steve@sakoman.com>