tests: fix non-portable iconv invocation

tests: fix non-portable iconv invocation

The iconv that comes with a FreeBSD 11.2-RELEASE-p2 box I have access
to doesn't support the SHIFT-JIS encoding. Guard a test added in
e92d62253 ("convert: add round trip check based on
'core.checkRoundtripEncoding'", 2018-04-15) first released with Git
v2.18.0 with a prerequisite that checks for its availability.

The iconv command is in POSIX, and we have numerous tests
unconditionally relying on its ability to convert ASCII, UTF-8 and
UTF-16, but unconditionally relying on the presence of more obscure
encodings isn't portable.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>