Hi all!

As suggested by mirai off-list, it would be nice to have a test that
would have caught the issue. How do people feel about something along
the following patch? The idea is to use namespaces in the dovecot config
to declare the INBOX and another additional mailbox.

The bug is quite obscure and not really about dovecot itself, so I don't
think adding such a test is strictly necessary, maybe more tests for the
configuration macro would be good instead. But I figured expanding the
dovecot system test can't hurt. Let me know if you agree and I'll format
it properly.