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_true_false.vim b/src/testdir/test_true_false.vim
index 4e14d55..37b0997 100644
--- a/src/testdir/test_true_false.vim
+++ b/src/testdir/test_true_false.vim
@@ -153,3 +153,5 @@
     call assert_equal('', r, 'result for ' . v . ' is not "" but ' . r)
   endfor
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab