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_vartabs.vim b/src/testdir/test_vartabs.vim
index 95fffe0..7bdcde5 100644
--- a/src/testdir/test_vartabs.vim
+++ b/src/testdir/test_vartabs.vim
@@ -147,9 +147,7 @@
 endfunc
 
 func Test_vartabs_breakindent()
-  if !exists("+breakindent")
-    return
-  endif
+  CheckOption breakindent
   new
   %d