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_retab.vim b/src/testdir/test_retab.vim
index f11a32b..b792da5 100644
--- a/src/testdir/test_retab.vim
+++ b/src/testdir/test_retab.vim
@@ -1,4 +1,5 @@
 " Test :retab
+
 func SetUp()
   new
   call setline(1, "\ta  \t    b        c    ")
@@ -75,3 +76,5 @@
   call assert_fails('retab -1',  'E487:')
   call assert_fails('retab! -1', 'E487:')
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab