patch 9.0.0380: deleting files in tests is a hassle

Problem:    Deleting files in tests is a hassle.
Solution:   Use the new 'D' flag of writefile().

src/vim9cmds.c

@@ -1705,6 +1705,27 @@ get_defer_var_idx(cctx_T *cctx)
     return dfunc->df_defer_var_idx;
 }
 
+/*
+ * Get the local variable index for deferred function calls.
+ * Reserve it when not done already.
+ * Returns zero for failure.
+ */
+    int
+get_defer_var_idx(cctx_T *cctx)
+{
+    dfunc_T	*dfunc = ((dfunc_T *)def_functions.ga_data)
+					       + cctx->ctx_ufunc->uf_dfunc_idx;
+    if (dfunc->df_defer_var_idx == 0)
+    {
+	lvar_T *lvar = reserve_local(cctx, (char_u *)"@defer@", 7,
+							    TRUE, &t_list_any);
+	if (lvar == NULL)
+	    return 0;
+	dfunc->df_defer_var_idx = lvar->lv_idx + 1;
+    }
+    return dfunc->df_defer_var_idx;
+}
+
 /*
  * Compile "defer func(arg)".
  */