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/t9700-perl-git.sh

@@ -6,6 +6,11 @@
 test_description='perl interface (Git.pm)'
 . ./test-lib.sh
 
+if ! test_have_prereq PERL; then
+	say 'skipping perl interface tests, perl not available'
+	test_done
+fi
+
 perl -MTest::More -e 0 2>/dev/null || {
 	say "Perl Test::More unavailable, skipping test"
 	test_done