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_messages.vim b/src/testdir/test_messages.vim
index f26b8c0..c575672 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -94,9 +94,8 @@
 endfunc
 
 func Test_mode_message_at_leaving_insert_by_ctrl_c()
-  if !has('terminal') || has('gui_running')
-    return
-  endif
+  CheckFeature terminal
+  CheckNotGui
 
   " Set custom statusline built by user-defined function.
   let testfile = 'Xtest.vim'
@@ -126,9 +125,8 @@
 endfunc
 
 func Test_mode_message_at_leaving_insert_with_esc_mapped()
-  if !has('terminal') || has('gui_running')
-    return
-  endif
+  CheckFeature terminal
+  CheckNotGui
 
   " Set custom statusline built by user-defined function.
   let testfile = 'Xtest.vim'