t9822: use prereq to check for ISO-8859-1 support

t9822: use prereq to check for ISO-8859-1 support

Tests in t9822 depend on filesystem support for ISO-8859-1 encoding. We
thus have a block of code that acts as a prerequisite -- if we fail to
write a file with an ISO-8859-1-encoded file name to disk then we skip
all tests.

When the prerequisite fails though we end up printing an error message
to stderr, which breaks the TAP format. Fix this by converting the code
to a proper prerequisite, which handles output redirection for us.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>