Test what happens if unencrypted group has key-contact

Due to bug #7088 some users now have unencrypted groups with one key-contact.

We need a test adding key-contact to such group and see what happens when sending a message in two cases:
- If key-contact has a different address from all the key-contacts (this is what happens when buggy group is just created)
- If key-contact has the same address as one of the address-contacts (this happens to the buggy group eventually when we receive a reply, apparently)

At least no assertion should fail, the message should be added to SMTP queue, recipient address should not be duplicated in the list of SMTP recipients. Maybe we should remove the key-contact silently to recover such groups.