CA-394882: avoid error on tasks that are not ours

The previous unit test step has created some tasks, and deleted them when completed.
However it didn't look at events.
The next step was waiting for some newly created tasks to complete and looking at events.
But it was processing all events, even those that it wasn't watching for.
These events may have referred to tasks that were already deleted.

Fix the code to only query the state of the tasks that we are waiting for
(if these go missing, it is a bug).

Signed-off-by: Edwin Török <edwin.torok@cloud.com>

ocaml/xenopsd/test/test.ml

@@ -91,7 +91,8 @@ let wait_for_tasks id =
     List.iter
       (function
         | Dynamic.Task id' ->
-            if task_ended dbg id' then ids := StringSet.remove id' !ids
+            (* ignore events on tasks that are not ours, they may have been deleted *)
+            if StringSet.mem id' !ids && task_ended dbg id' then ids := StringSet.remove id' !ids
         | _ ->
             ()
         )