patch 8.2.4212: window title test still fails in some configurations

Problem:    Window title test still fails in some configurations.
Solution:   Explicitly set the 'title' option.

src/testdir/test_functions.vim

@@ -1637,6 +1637,7 @@ func Test_setbufvar_keep_window_title()
   endif
 
   let lines =<< trim END
+      set title
       edit Xa.txt
       let g:buf = bufadd('Xb.txt')
       inoremap <F2> <C-R>=setbufvar(g:buf, '&autoindent', 1) ?? ''<CR>