feat: perform background fetch from all transports

With I/O stopped, `background_fetch()` connected only to the transport of `configured_addr`
and we now instead fan out to all transports in a controlled loop.
If a first transport finished fetching new messages
cancel all other attempts and return.

This is meant to address the problem that amzd described
where a profile with one functioning and one hanging transport,
shows the first notification, then hangs 15 seconds waiting for the hanging transport.
meanwhile a second NSE arrives and dies, and the second message is not notified
or only generically.

Also drop the quota check from this background fetch path:
its result is in-memory only, discarded when the iOS notification service exits,
and the regular scheduler fetching refreshes it every 60s anyway.
Moreover, quota errors/running full is pretty rare
since relays generally automatically stay under quota these days.
It's another round trip for each transport of each profile and simply not necessary.

Also adds previously missing online tests.

deltachat-jsonrpc/src/api.rs

@@ -2038,6 +2038,14 @@ impl CommandApi {
         Ok(())
     }
 
+    /// Waits until all transports are idle or failed and no background work is left.
+    /// Never returns unless I/O is started. Must ONLY be used by tests.
+    async fn wait_for_all_work_done(&self, account_id: u32) -> Result<()> {
+        let ctx = self.get_context(account_id).await?;
+        ctx.wait_for_all_work_done().await;
+        Ok(())
+    }
+
     /// Get the current connectivity, i.e. whether the device is connected to the IMAP server.
     /// One of:
     /// - DC_CONNECTIVITY_NOT_CONNECTED (1000): Show e.g. the string "Not connected" or a red dot