patch 8.2.0866: not enough tests for buffer writing

Problem:    Not enough tests for buffer writing.
Solution:   Add more tests. Use CheckRunVimInTerminal in more places.
            (Yegappan Lakshmanan, closes #6167)
diff --git a/src/testdir/test_messages.vim b/src/testdir/test_messages.vim
index 832762a..5d6959b 100644
--- a/src/testdir/test_messages.vim
+++ b/src/testdir/test_messages.vim
@@ -1,5 +1,6 @@
 " Tests for :messages, :echomsg, :echoerr
 
+source check.vim
 source shared.vim
 source term_util.vim
 source view_util.vim
@@ -172,9 +173,7 @@
 
 " Test more-prompt (see :help more-prompt).
 func Test_message_more()
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot run vim in terminal'
-  endif
+  CheckRunVimInTerminal
   let buf = RunVimInTerminal('', {'rows': 6})
   call term_sendkeys(buf, ":call setline(1, range(1, 100))\n")
 
@@ -265,9 +264,7 @@
 endfunc
 
 func Test_ask_yesno()
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot run vim in terminal'
-  endif
+  CheckRunVimInTerminal
   let buf = RunVimInTerminal('', {'rows': 6})
   call term_sendkeys(buf, ":call setline(1, range(1, 2))\n")