patch 8.2.3630: printf() with %S does not handle multi-byte correctly

Problem:    Printf() with %S does not handle multi-byte correctly.
Solution:   Count cells instead of bytes. (closes #9169, closes #7486)
diff --git a/src/testdir/test_expr.vim b/src/testdir/test_expr.vim
index 080eee7..9700974 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -297,6 +297,11 @@
   call assert_equal('🐍', printf('%.2S', '🐍🐍'))
   call assert_equal('', printf('%.1S', '🐍🐍'))
 
+  call assert_equal('[    あいう]', printf('[%10.6S]', 'あいうえお'))
+  call assert_equal('[  あいうえ]', printf('[%10.8S]', 'あいうえお'))
+  call assert_equal('[あいうえお]', printf('[%10.10S]', 'あいうえお'))
+  call assert_equal('[あいうえお]', printf('[%10.12S]', 'あいうえお'))
+
   call assert_equal('1%', printf('%d%%', 1))
 endfunc