Enhance test data generation facility

[cbrnr]

Many tests require (artificial) testing data. At the moment, many tests create these data for themselves. It would be nice if we had one place that generates testing data for all tests - we can use generate_testdata.py for this purpose.

It would also be awesome if we could get a realistic leadfield matrix from a head model to generate realistic EEG-like testing data.

Finally, we could also (optionally) use our example data set as testing data for (optional) tests.