tests: Let xorriso fixely assume UTF-8 as local character set

tests: Let xorriso fixely assume UTF-8 as local character set

The iso9660_test fails if the effective locale is not UTF-8. This happens
because xorriso needs to convert file names and FSLABEL to UCS-2 when
preparing a Joliet tree. The grub-fs-tester obviously intends to use UTF-8
as character set, but xorriso assumes by default the result of nl_langinfo(3)
with item CODESET. So, override the result of nl_langinfo(CODESET) by options
of xorriso -as mkisofs.

Signed-off-by: Thomas Schmitt <scdbackup@gmx.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>