patch 9.0.0538: manually deleting test temp files

Problem:    Manually deleting test temp files.
Solution:   Add the 'D' flag to writefile().

src/testdir/test_execute_func.vim

@@ -183,13 +183,12 @@ func Test_win_execute_on_startup()
       silent tabedit Xfile3
       autocmd VimEnter * win_execute(id, 'close')
   END
-  call writefile(lines, 'XwinExecute')
+  call writefile(lines, 'XwinExecute', 'D')
   let buf = RunVimInTerminal('-p Xfile1 -Nu XwinExecute', {})
 
   " this was crashing on exit with EXITFREE defined
   call StopVimInTerminal(buf)
 
-  call delete('XwinExecute')
   call delete('Xfile1')
 endfunc