Better structure tests between zppy & zppy-interfaces

Rethink how testing is done in `zppy` & `zppy-interfaces`. `zppy` should simply test that it can invoke `zppy-interfaces global-time-series`. `zppy-interfaces` however should test that Global Time Series plots actually generate correctly.

Maybe we could have `zppy` test that `zppy` is passing in the right parameters and then have `zppy-interfaces` actually do the image comparisons of the plots, using pre-generated `ts` and `mpas_analysis` output.