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_channel.vim b/src/testdir/test_channel.vim
index ae7e301..065e8b4 100644
--- a/src/testdir/test_channel.vim
+++ b/src/testdir/test_channel.vim
@@ -1348,9 +1348,7 @@
 " This caused a crash, because messages were handled while peeking for a
 " character.
 func Test_exit_cb_wipes_buf()
-  if !has('timers')
-    return
-  endif
+  CheckFeature timers
   set cursorline lazyredraw
   call test_override('redraw_flag', 1)
   new