`test_qr_securejoin_broadcast` is flaky

There were two test failures in a CI run today (in the MacOS runner), `test_dont_move_sync_msgs` and `test_qr_securejoin_broadcast`. My investigation so far:
- `test_qr_securejoin_broadcast`:
It fails because the chat doesn't show the member addition for Alice's second device.
There is the line `src/receive_imf.rs:3620: No-op broadcast addition (TRASH)` in the logs, i.e. Alice2 doesn't show the member addition message because it already had Bob in the broadcast. The reason might be https://github.com/chatmail/core/pull/7578/, though it clearly sends the member-addition first and then the sync message. Maybe the sync message somehow overtook the member-addition message, but I'm not sure how.
- `test_dont_move_sync_msgs`: **Edit:** This one is fixed
It sends two sync messages, and then tests that both of them stay in the Inbox, rather than being moved. In the logs, I can see that it correctly sends both sync messages, and doesn't move them. But then, when direct_imap checks the server, there is only one message. One reason might be a race condition in the server, that the sent message didn't reach the inbox yet by the time we check.

Unfortunately, I didn't manage to reproduce either test failure locally.

[Logs](https://github.com/user-attachments/files/24716969/flaky-tests.txt)