[iot] testing: possible bug in the cleanup code


iot/api-client/manager/manager_test.py

```
Traceback (most recent call last):
  File "/workspace/iot/api-client/manager/manager_test.py", line 57, in clean_up_registries
    test_date = datetime.datetime.utcfromtimestamp(int(time_str))
ValueError: invalid literal for int() with base 10: 'b32153211df0'
```
