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_startup.vim b/src/testdir/test_startup.vim
index fe93eab..0069628 100644
--- a/src/testdir/test_startup.vim
+++ b/src/testdir/test_startup.vim
@@ -673,9 +673,7 @@
 endfunc
 
 func Test_start_with_tabs()
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot make screendumps'
-  endif
+  CheckRunVimInTerminal
 
   let buf = RunVimInTerminal('-p a b c', {})
   call VerifyScreenDump(buf, 'Test_start_with_tabs', {})
@@ -783,9 +781,7 @@
 
 " Test for specifying a non-existing vimrc file using "-u"
 func Test_missing_vimrc()
-  if !CanRunVimInTerminal()
-    throw 'Skipped: cannot run vim in terminal'
-  endif
+  CheckRunVimInTerminal
   let after =<< trim [CODE]
     call assert_match('^E282:', v:errmsg)
     call writefile(v:errors, 'Xtestout')