patch 8.2.0270: some code not covered by tests

Problem:    Some code not covered by tests.
Solution:   Add test cases. (Yegappan Lakshmanan, closes #5649)
diff --git a/src/testdir/test_writefile.vim b/src/testdir/test_writefile.vim
index f616980..5449071 100644
--- a/src/testdir/test_writefile.vim
+++ b/src/testdir/test_writefile.vim
@@ -215,9 +215,7 @@
   close!
 
   call assert_fails('w > Xtest', 'E494:')
-
-  call assert_fails('w > Xtest', 'E494:')
-
+ 
   " Try to overwrite a directory
   if has('unix')
     call mkdir('Xdir1')