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_tab.vim b/src/testdir/test_tab.vim
index b8e8dfe..da630f0 100644
--- a/src/testdir/test_tab.vim
+++ b/src/testdir/test_tab.vim
@@ -88,3 +88,5 @@
   set sts=0 sw=0 backspace& nosmarttab
   bwipe!
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab