`test_qr_securejoin_broadcast` is flaky

[Hocuri]

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 Sync broadcast subscribers list #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

[link2xt]

Fix for test_dont_move_sync_msgs is in #7787