test: Fix flaky test_transport_synchronization (hopefully)

[Hocuri]

Fix #7835 (hopefully).

The problem was most probably:

• ac1_clone receives the sync message, sends TRANSPORTS_MODIFIED event, and launches a task that will restart IO
• After IO was stopped, but before it is started again, ac1_clone.add_transport_from_qr(qr) is called
• this check fails:

        ensure!(
            !self.scheduler.is_running().await,
            "cannot configure, already running"
        );

There are multiple possible ways to fix this; if you think something else should be done, you can also just push it.

[link2xt]

CI is failing with timeout waiting for I/O start.