support.task_queue: clear done queue after cancellation.

Since .cancel() waits until all live tasks are done, it is likely
that the done queue will contain some cancelled tasks. If the done
queue is not cleared, then the next poll()/wait_one()/wait_all() call
will cancel the caller, which usually silently terminates it. In any
case, semantically .cancel() discards the results of all tasks in
the queue even if they completed before they were cancelled.

This bug was introduced in commit a7d041f.

Fixes #202.

Co-authored-by: Filipe Laíns <lains@archlinux.org>

software/glasgow/support/task_queue.py

@@ -40,6 +40,7 @@ async def cancel(self):
             task.cancel()
         if self._live:
             await asyncio.wait(self._live, return_when=asyncio.ALL_COMPLETED)
+        self._done.clear()
 
     async def poll(self):
         """