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_tabpage.vim b/src/testdir/test_tabpage.vim
index 80cc8e5..a9bf7a1 100644
--- a/src/testdir/test_tabpage.vim
+++ b/src/testdir/test_tabpage.vim
@@ -593,9 +593,7 @@
 endfunc
 
 func Test_tabpage_cmdheight()
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot make screendumps'
-  endif
+  CheckRunVimInTerminal
   call writefile([
         \ 'set laststatus=2',
         \ 'set cmdheight=2',