[thread-cert] simulator: mark virtual-time node 'awake' whenever an event was sent to it (#12333)

This marks a simulation node as 'awake' whenever an event is sent to
it. This is required because the event will induce processing on the
node followed by an alarm-event sent back to the simulator.  The
simulator needs to wait to properly catch this final alarm-event;
otherwise, this alarm-event may be mistakenly processed as
confirmation of a newer event that follows later, in case the OT node
process is lagging in processing as may happen typically in CI
environments.

Also the case of unknown event type is now raised as an exception, to
avoid undetected errors in the simulation. This could cause an event
to be not sent to the node at all while the node's time is updated by
the simulator (using `self.devices[port]['time'] = event_time`) as if
the node is up to date on the latest virtual time, which it isn't,
because an event is not sent to the node.

Also fixes a potential undefined local variable 'data' flagged by the
IDE.

tests/scripts/thread-cert/simulator.py

@@ -528,7 +528,8 @@ class VirtualTime(BaseSimulator):
 
         if len(event) == 5:
             event_time, sequence, port, type, datalen = event
-            dbg_print("Pop event: ", event_time, port, type, datalen)
+            data = b''
+            dbg_print("Pop event: ", event_time, port, type, datalen, "(no data)")
         else:
             event_time, sequence, port, type, datalen, data = event
             dbg_print(
@@ -562,6 +563,10 @@ class VirtualTime(BaseSimulator):
         elif type == self.OT_SIM_EVENT_UART_WRITE:
             message += data
             self._send_message(message, port)
+        else:
+            raise NotImplementedError(f'Unknown event type: {type}')
+
+        self.awake_devices.add(port)
 
     def sync_devices(self):
         self.current_time = self._pause_time