patch 9.0.0538: manually deleting test temp files

Problem:    Manually deleting test temp files.
Solution:   Add the 'D' flag to writefile().
diff --git a/src/testdir/test_exit.vim b/src/testdir/test_exit.vim
index a7596e1..9e9370e 100644
--- a/src/testdir/test_exit.vim
+++ b/src/testdir/test_exit.vim
@@ -117,13 +117,12 @@
   CheckNotAsan
   CheckNotValgrind
 
-  call writefile([":au VimLeave * call writefile(['l = ' .. v:exiting], 'Xtestout')", ":tabnew", "q:"], 'Xscript', 'b')
+  call writefile([":au VimLeave * call writefile(['l = ' .. v:exiting], 'Xtestout')", ":tabnew", "q:"], 'Xscript', 'bD')
 
   if RunVim([], [], '<Xscript')
     call assert_equal(1, v:shell_error)
     call assert_equal(['l = 1'], readfile('Xtestout'))
   endif
-  call delete('Xscript')
   call delete('Xtestout')
 endfun