tests: Skip tests in a way that makes sense under TAP

SKIP messages are now part of the TAP plan. A TAP harness now knows
why a particular test was skipped and can report that information. The
non-TAP harness built into Git's test-lib did nothing special with
these messages, and is unaffected by these changes.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t5302-pack-index.sh b/t/t5302-pack-index.sh
index 4360e77..fb3a270 100755
--- a/t/t5302-pack-index.sh
+++ b/t/t5302-pack-index.sh
@@ -74,7 +74,7 @@
 then
 	test_set_prereq OFF64_T
 else
-	say "skipping tests concerning 64-bit offsets"
+	say "# skipping tests concerning 64-bit offsets"
 fi
 
 test_expect_success OFF64_T \