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_expand_func.vim b/src/testdir/test_expand_func.vim
index 454d76f..c9e802a 100644
--- a/src/testdir/test_expand_func.vim
+++ b/src/testdir/test_expand_func.vim
@@ -56,14 +56,13 @@
     " comment here
     let g:stack_value = expand('<stack>')
   END
-  call writefile(lines, 'Xstack')
+  call writefile(lines, 'Xstack', 'D')
   source Xstack
   call assert_match('\<Xstack\[2\]$', g:stack_value)
   unlet g:stack_value
-  call delete('Xstack')
 
   if exists('+shellslash')
-    call mkdir('Xshellslash')
+    call mkdir('Xshellslash', 'R')
     let lines =<< trim END
       let g:stack1 = expand('<stack>')
       set noshellslash
@@ -83,7 +82,6 @@
       unlet g:stack2
       unlet g:stack3
     endfor
-    call delete('Xshellslash', 'rf')
   endif
 endfunc