auto-t: add clear_events() to IWD class

The wait_for_event() function allows past events to cause this
function to return immediately. This behavior is known, and
relied on for some tests. But in some cases you want to only
handle _new_ events, so we need a way to clear out prior events.

autotests/util/iwd.py

@@ -299,6 +299,9 @@ class StationDebug(IWDDBusAbstract):
 
         return False
 
+    def clear_events(self):
+        self._events = []
+
     def wait_for_event(self, event, timeout=10):
         return ctx.non_block_wait(self._poll_event, timeout, event,
                                     exception=TimeoutError("waiting for event"))
@@ -859,6 +862,9 @@ class Device(IWDDBusAbstract):
     def wait_for_event(self, event, timeout=10):
         self._station_debug.wait_for_event(event, timeout)
 
+    def clear_events(self):
+        self._station_debug.clear_events()
+
     def event_ocurred(self, event):
         return self._station_debug.event_ocurred(event)