patch 8.0.1267: Test_swap_group may leave file behind

Problem:    Test_swap_group may leave file behind.
Solution:   Add a try/finally.
diff --git a/src/testdir/test_undo.vim b/src/testdir/test_undo.vim
index bda7a44..4ea1c9b 100644
--- a/src/testdir/test_undo.vim
+++ b/src/testdir/test_undo.vim
@@ -190,6 +190,7 @@
 endfunc
 
 func Test_undo_write()
+  call delete('Xtest')
   split Xtest
   call feedkeys("ione one one\<Esc>", 'xt')
   w!