Test `test_verified_group_vs_delete_server_after` is flaky

[link2xt]

Attached is a log from CI:
flaky-test.txt

Maybe similar issue to #5191 but better understand what exactly happened.

[iequidoo]

It happened also to me a couple of times, but i didn't understand the reason. In the log it looks like ac2 doesn't receive the vg-member-added message

[link2xt]

Maybe this message is delayed and arrives later?

[iequidoo]

Apparently it arrives immediately, but we still have some race in IMAP handling:
https://github.com/deltachat/deltachat-core-rust/actions/runs/7630255470/job/20797452090?pr=5209
After IO has been restarted, vg-member-added is received and the debug assertion fired.

[iequidoo]

As far as i can tell from the log, there's a race in Session::idle() somewhere between the call to self.server_sent_unsolicited_exists(context) and handle.wait_with_timeout(IDLE_TIMEOUT). So, either it's a problem in async_imap not interrupting the wait_for_timeout() call if new unsolicited responses have arrived or the server implementation not interrupting IMAP IDLE if new emails have arrived just before a receipt of the IDLE command.