vim-patch:9.1.1974: tests: missing cleanup in Test_bwipe_during_save()

Problem:  tests: missing cleanup in Test_bwipe_during_save().
Solution: Reset &charconvert.

closes: vim/vim#18908

https://github.com/vim/vim/commit/353103e59afddbfae8432e715217b809e8e15998

test/old/testdir/test_buffer.vim

@@ -232,6 +232,7 @@ func Test_bwipe_during_save()
   set charconvert=execute('%bw!')
   call assert_fails('write ++enc=lmao boom', 'E937:')
 
+  set charconvert&
   %bwipe
 endfunc