worker: do repeatedly remove already awaited tasks from subtask list

Plain TaskBase.wait() always uses the **whole** list of all subtasks as
the set of subtasks that should be awaited.  Therefore, it will try to
await even tasks that were already awaited in some previous
TaskBase.wait() call and are no longer present in self._subtask_list
which subsequently raise the ValueError shown below.

Keep the set of already awaited tasks and skip their IDs when marking
newly awaited subtasks as finished.

Fixes the following traceback:
```
Traceback (most recent call last):
  File "/src/kobo/kobo/worker/taskmanager.py", line 423, in run_task
    task.run()
  File "/src/osh/worker/tasks/task_errata_diff_build.py", line 55, in run
    self.wait()
  File "/src/kobo/kobo/worker/task.py", line 147, in wait
    self._subtask_list.remove(i)
ValueError: list.remove(x): x not in list
```

kobo/worker/task.py

@@ -32,7 +32,8 @@ def __init__(self, hub, conf, task_id, args):
         self._task_info = self.hub.worker.get_task(self.task_id)
         self._task_manager = None  # created by taskmanager (only for foreground tasks)
         self._args = args
-        self._subtask_list = []
+        self._finished_subtasks = set()
+        self._running_subtask_list = []
         self.result = ""
 
     @property
@@ -71,7 +72,7 @@ def args(self):
     @property
     def subtask_list(self):
         # deepcopy to prevent modification
-        return copy.deepcopy(self._subtask_list)
+        return copy.deepcopy(self._running_subtask_list)
 
     def run(self):
         """Run the task."""
@@ -95,7 +96,7 @@ def spawn_subtask(self, method, args, label="", priority = None):
             raise RuntimeError("Foreground tasks can't spawn subtasks.")
 
         subtask_id = self.hub.worker.create_subtask(label, method, args, self.task_id, priority)
-        self._subtask_list.append(subtask_id)
+        self._running_subtask_list.append(subtask_id)
         return subtask_id
 
     def wait(self, subtasks=None):
@@ -126,13 +127,19 @@ def wait(self, subtasks=None):
             signal.pause()
             # wake up on signal to check the status
 
+        # do not await subtasks that were already processed in previous wait calls
+        finished = [x for x in finished if x not in self._finished_subtasks]
+
         # remove finished subtasks from the list, check results
         fail = False
         for i in finished:
             state = self.hub.worker.get_task(i)
             if state['state'] != TASK_STATES['CLOSED']:
                 fail = True
-            self._subtask_list.remove(i)
+            self._running_subtask_list.remove(i)
+
+        # mark finished subtasks as processed
+        self._finished_subtasks.update(finished)
 
         if fail:
             print("Failing because of at least one subtask hasn't closed properly.")