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_ex_undo.vim b/src/testdir/test_ex_undo.vim
index 44feb36..48a87df 100644
--- a/src/testdir/test_ex_undo.vim
+++ b/src/testdir/test_ex_undo.vim
@@ -17,3 +17,5 @@
   call assert_equal(1, line('$'))
   quit!
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab