tests: Fix WaitForOneFromTask constructor parameter introspection

Strings are iterable, so the check in WaitForOneFromTask was
effectively useless. This lead to TypeErrors (caught by transition
notifier) when it tried to find None in 'task1'.

Change-Id: I3e91029f7b7cb960d6ca84490de76b5e044bc238
Merge "tests: Fix WaitForOneFromTask constructor parameter introspection"