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_exists_autocmd.vim b/src/testdir/test_exists_autocmd.vim
index 7e44a72..58ad9b5 100644
--- a/src/testdir/test_exists_autocmd.vim
+++ b/src/testdir/test_exists_autocmd.vim
@@ -24,3 +24,5 @@
   edit testfile2.test
   call assert_false(exists("#BufEnter#<buffer>"))
 endfunction
+
+" vim: shiftwidth=2 sts=2 expandtab