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)

src/testdir/test_set.vim

@@ -25,3 +25,5 @@ function Test_set_add()
 
   let &wig = wig_save
 endfunction
+
+" vim: shiftwidth=2 sts=2 expandtab