patch 8.2.1432: various inconsistencies in test files

Problem:    Various inconsistencies in test files.
Solution:   Add modelines where they were missing.  Use Check commands instead
            of silently skipping over tests.  Adjust indents and comments.
            (Ken Takata, closes #6695)
diff --git a/src/testdir/test_listchars.vim b/src/testdir/test_listchars.vim
index 987ed66..d7658bf 100644
--- a/src/testdir/test_listchars.vim
+++ b/src/testdir/test_listchars.vim
@@ -204,3 +204,5 @@
   enew!
   set listchars& ff&
 endfunction
+
+" vim: shiftwidth=2 sts=2 expandtab