feat: replace event channel with broadcast channel

This makes `EventTracker` receive events immediately
instead of being moved from event emitter to event tracker
by a task spawned from `TestContext::new_internal`.

This makes `EventTracker.clear_events` reliable
as it is guaranteed to remove all events emitted
by the time it is called rather than only events
that have been moved already.

deltachat-ffi/src/lib.rs

@@ -4922,7 +4922,9 @@ mod jsonrpc {
         }
 
         let account_manager = &*account_manager;
-        let cmd_api = deltachat_jsonrpc::api::CommandApi::from_arc(account_manager.inner.clone());
+        let cmd_api = block_on(deltachat_jsonrpc::api::CommandApi::from_arc(
+            account_manager.inner.clone(),
+        ));
 
         let (request_handle, receiver) = RpcClient::new();
         let handle = RpcSession::new(request_handle, cmd_api);