fix permanently hiding of one-to-one chats after secure-join (#2791)

* test one-to-one chats on setup-contact/secure-join

only one chat is created after scanning a QR code:

- on setup-contact, one-to-ones are created on both sided

- on secure-join, the joined group chat is created;
  one-to-ones are not created intitally,
  but should become visible on receiving messages

* make sure, Alice creates the chat with Bob on setup-contact

not totally sure if that change in #2508 was on-purpose,
however, all yet released versions
did create the one-to-one chat also on the Inviter's (Alice) side,
so, let's stay with that,
i do not see many reasons to change that.

* unblock hidden (Blocked::Yes) one-to-one chats

one-to-one chats may be hidden by secure-join,
in case someone later writes a message to it
(not unlikely), the chat needs to be shown.

before, messages are just not shown,
the corresponding chat did not appear.

the 'Blocked' wording of a 'Chat' must not be mixed with the
'Blocking' of a contact. 'Chat-Blocking' is mostly a visibility thing,
that may change as messages come in.

this change should not affect _really_ blocked contacts -
they are filtered out already before
and their messages are usually not even downloaded.
also, before allow_creation is checked,
that may disallow chat creation for show_emails reasons.

all in all, it just does the same
as if the user has manualy deleted the chat before and it would be created.

* simplify test

src/chat.rs

@@ -266,7 +266,7 @@ impl ChatId {
     /// Updates chat blocked status.
     ///
     /// Returns true if the value was modified.
-    async fn set_blocked(self, context: &Context, new_blocked: Blocked) -> Result<bool> {
+    pub(crate) async fn set_blocked(self, context: &Context, new_blocked: Blocked) -> Result<bool> {
         if self.is_special() {
             bail!("ignoring setting of Block-status for {}", self);
         }