test: add more obsmarker tests for pruning scenarios The obsfate output in cases of pruning is not ideal right now, add some tests so have these scenarios around.

template: better prune support in obsfate successorssets don't returns good results for pruned commit, add a workaround for simple cases. A proper fix would require a large rework of successorssets algorithm, I will send a separate series for this refactoring.

template: compute dates in obsfatedate Extract the dates from obsmarkers. Compute the min and max date from the obsmarker range list.