tests: fix locale issue on non-Debian linux
tests: fix locale issue on non-Debian linux

The C.UTF-8 locale is Debian-specific and causes issues on other
platforms.

To avoid setlocale error messages, the LC_ALL export will default on
C.UTF-8, the system utf-8 locale if unavailable, and on C if no better
choice is available.

Signed-off-by: Lucas Bajolet <7ee927989f658093d1603ebdd77063d48a82a2fc@gmail.com>

privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit,privat/nit