Also fixing unused dataset_id=None in _callFUT in
Test__get_gcd_dataset_id and Test__get_gcd_dataset_id.

Also adding the relative ordering parts to Test__determine_default_dataset_id.

Tests previously were mixing the logic of the 4 possible environments
while the simple ordering of them in set_default_dataset_id() and
now _determine_default_dataset_id().

This separates that logic and makes sure each environ helper is called
in a specific order.