fix: Call send_sync_msg() only from the SMTP loop (#5780)

`Context::send_sync_msg()` mustn't be called from multiple tasks in parallel to avoid sending the
same sync items twice because sync items are removed from the db only after successful
sending. Let's guarantee this by calling `send_sync_msg()` only from the SMTP loop. Before
`send_sync_msg()` could be called in parallel from the SMTP loop and another task doing
e.g. `chat::sync()` which led to `test_multidevice_sync_chat` being flaky because of events
triggered by duplicated sync messages.

src/test_utils.rs

@@ -1104,6 +1104,7 @@ pub(crate) async fn mark_as_verified(this: &TestContext, other: &TestContext) {
 /// Pops a sync message from alice0 and receives it on alice1. Should be used after an action on
 /// alice0's side that implies sending a sync message.
 pub(crate) async fn sync(alice0: &TestContext, alice1: &TestContext) {
+    alice0.send_sync_msg().await.unwrap();
     let sync_msg = alice0.pop_sent_msg().await;
     let no_msg = alice1.recv_msg_opt(&sync_msg).await;
     assert!(no_msg.is_none());