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>

t/t9129-git-svn-i18n-commitencoding.sh

@@ -23,7 +23,7 @@ if test -n "$a_utf8_locale"
 then
 	test_set_prereq UTF8
 else
-	say "UTF-8 locale not available, some tests are skipped"
+	say "# UTF-8 locale not available, some tests are skipped"
 fi
 
 compare_svn_head_with () {