run_constraints should not introduce a cyclic dependency, but tests should be deferred

[wolfv]

Currently, we use all the dependencies for the topological sorting, including the ones from run and run_constraints.

However, these should only be "weak" dependencies for the sorting since they only introduce a dependency that is needed at test time.

I think we should ignore them when building, but defer testing if necessary to the time when all dependencies are available. If possible, we should test asap, though.