t/test-lib: wire up NO_ICONV prerequisite

t/test-lib: wire up NO_ICONV prerequisite

The iconv library is used by Git to reencode files, commit messages and
other things. As such it is a rather integral part, but given that many
platforms nowadays use UTF-8 everywhere you can live without support for
reencoding in many situations. It is thus optional to build Git with
iconv, and some of our platforms wired up in "config.mak.uname" disable
it. But while we support building without it, running our test suite
with "NO_ICONV=Yes" causes many test failures.

Wire up a new test prerequisite ICONV that gets populated via our
GIT-BUILD-OPTIONS. Annotate failing tests accordingly.

Note that this commit does not do a deep dive into every single test to
assess whether the failure is expected or not. Most of the tests do
smell like the expected kind of failure though.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Taylor Blau <me@ttaylorr.com>