tests/core: Assume C.UTF-8 if locale isn't found
tests/core: Assume C.UTF-8 if locale isn't found

When building with musl there's no locale command, also its default
locale is C.UTF-8, so just get C.UTF-8 if we can't find locale.

Signed-off-by: Alex Kiernan <4b5f5be05eafb72e9d17316d31dbc865680e1023@gmail.com>

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree