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_flatten.vim b/src/testdir/test_flatten.vim
index 9908661..34431f8 100644
--- a/src/testdir/test_flatten.vim
+++ b/src/testdir/test_flatten.vim
@@ -1,4 +1,5 @@
 " Test for flatting list.
+
 func Test_flatten()
   call assert_fails('call flatten(1)', 'E686:')
   call assert_fails('call flatten({})', 'E686:')
@@ -79,3 +80,5 @@
   call assert_equal([1, 2, 1, 2], flatten(l:x, 2))
   call assert_equal([2, l:x], l:y)
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab