Add subtasks dependency

[kostka-dev]

In some problems, often subtasks depend on each other,

e.g. in the following case

1 | A, B <= 20 |
2 | A <= 20 |
3 | A <= 500 |
4 | B <= 20 |
5 | No additional constraints. |

2 depends on 1, 3 depends on 1 and 2, 4 depends on 1, and 5 depends on all the others.

It should be possible to integrate this in a way that if there is a failing test in group 1, groups 2, 3, 4, 5 also fail, without needing to add the same test in all groups. Note that we don't want to actually run these tests again, just append their scores to the proper groups.

This probably should be added in config.yml of the problem, for instance.