Ensure test coverage of callbacks

[oyvindeide]

To be able to replace the OK callback we need to make sure it is properly tested.

The callback is responsible to load three main things:

1. Parameters defined with FORWARD_INIT, i.e. parameters that are not sampled by ert, but read from file after the forward model has completed (GEN_KW, FIELD and SURFACE, and possibly also GEN_PARAM).
2. Loading summary data, this amounts to checking if the ECLBASE file is present in the run path (if summary is defined) and loading the data. There is a lot of complexity involved in checking if the time map is in sync. We need to test this behavior, but it is probably covered enough. We would like this behavior to change into saving the time map for each realization.
3. Loading GEN_DATA, here we can load both binary and ascii data, but should investigate if both are being used, suspect only ascii is being used, if so this can be simplified.

Known tests: test_load_forward_model.py test_parameter_sample_types.py

Edit: definition of done: reviewed test coverage to make sure we are ready to reimplement.