patch 9.0.0767: too many delete() calls in tests

Problem:    Too many delete() calls in tests.
Solution:   Use deferred delete where possible.
diff --git a/src/testdir/test_tabpage.vim b/src/testdir/test_tabpage.vim
index ef265ed..e57a5dd 100644
--- a/src/testdir/test_tabpage.vim
+++ b/src/testdir/test_tabpage.vim
@@ -606,13 +606,12 @@
         \ 'echo "hello\nthere"',
         \ 'tabnext',
         \ 'redraw',
-	\ ], 'XTest_tabpage_cmdheight')
+	\ ], 'XTest_tabpage_cmdheight', 'D')
   " Check that cursor line is concealed
   let buf = RunVimInTerminal('-S XTest_tabpage_cmdheight', {'statusoff': 3})
   call VerifyScreenDump(buf, 'Test_tabpage_cmdheight', {})
 
   call StopVimInTerminal(buf)
-  call delete('XTest_tabpage_cmdheight')
 endfunc
 
 " Test for closing the tab page from a command window