ENH: Add automated test for synthetic data to make sure default targets can be created based on it.

[ChristianZimpelmann]

Is your feature request related to a problem?

When introducing a new input variable it can happen that we forget to include the variable in synthetic.py.

Describe the solution you would like to see

Add automated test which:

• uses synthetic.py to create a dataset
• runs compute_taxes_and_transfers with default targets

[MImmesberger]

Closed because this duplicates #733