autotests: Reset _exception after _wait_for_async_op

We need to reset self._exception after _wait_for_async_op raises an
exception, otherwise _wait_for_async_op will report that exception for
every future operation (this wasn't an issue when an exception always
meant that the test was failing and objects were torn down anyway)

autotests/util/iwd.py

@@ -99,7 +99,9 @@ class AsyncOpAbstract(object):
 
         self._is_completed = False
         if self._exception is not None:
-            raise self._exception
+            tmp = self._exception
+            self._exception = None
+            raise tmp
 
 
 class IWDDBusAbstract(AsyncOpAbstract):