tests: skip perl tests if NO_PERL is defined

These scripts all test git programs that are written in
perl, and thus obviously won't work if NO_PERL is defined.
We pass NO_PERL to the scripts from the building Makefile
via the GIT-BUILD-OPTIONS file.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -698,6 +698,8 @@ case $(uname -s) in
 	;;
 esac
 
+test -z "$NO_PERL" && test_set_prereq PERL
+
 # test whether the filesystem supports symbolic links
 ln -s x y 2>/dev/null && test -h y 2>/dev/null && test_set_prereq SYMLINKS
 rm -f y