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_exists.vim

@@ -1,4 +1,5 @@
 " Tests for the exists() function
+
 func Test_exists()
   augroup myagroup
       autocmd! BufEnter       *.my     echo "myfile edited"