test-lib: on FreeBSD, look for unzip(1) in /usr/local/bin/

test-lib: on FreeBSD, look for unzip(1) in /usr/local/bin/

Eric Wong reported that while FreeBSD has a /usr/bin/unzip, it uses
different semantics from those that are needed by Git's tests: When
passing the -a option to Info-Zip, it heeds the text attribute of the
.zip file's central directory, while FreeBSD's unzip ignores that
attribute.

The common work-around is to install Info-Zip on FreeBSD, into
/usr/local/bin/.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Tested-by: Eric Wong <e@80x24.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>