CodingGuidelines: explicitly allow "local" for test scripts

CodingGuidelines: explicitly allow "local" for test scripts

01d3a526 (t0000: check whether the shell supports the "local"
keyword, 2017-10-26) raised a test balloon to see if those who build
and test Git use a platform with a shell that lacks support for the
"local" keyword.  After two years, 7f0b5908 (t0000: reword comments
for "local" test, 2019-08-08) documented that "local" keyword, even
though is outside POSIX, is allowed in our test scripts.

Let's write it in the CodingGuidelines, too.  It might be tempting
to allow it in scripted Porcelains (we have avoided getting them
contaminiated by "local" so far), but they are on their way out and
getting rewritten in C.

Signed-off-by: Junio C Hamano <gitster@pobox.com>