[utils/tasktester] Make the task tester compatible with the new taskset feature

[nrybowski]

Is your feature request related to a problem? Please describe.
inginious.org already deploys unmerged features such as the new tasksets (See https://github.com/UCL-INGI/INGInious/tree/tasksets).

However, the task tester script relies on the current course format and fails with the newer taskset format. See:

Traceback (most recent call last):
  File "/inginious/./inginious-test-task", line 301, in <module>
    taskset_factory, task_factory = create_factories(local_fsp, task_dispensers, task_problem_types)
ValueError: too many values to unpack (expected 2)

Hence, courses currently available on inginious.org won't benefit from the CI task tester feature.