filter tests:: --filter-ancestor=-blacklist:blacklist ain't working. try to fix

add a test for this case

the problem: at the time the ancestor filter does its thing, the blacklist:blacklist target doesn't "realize" it has deps. My hazy understanding: we added blacklist:blacklist to the build graph, but didn't tell it to, uhm, recurse its deps. I think this change tells it to recurse its deps. And I guess that's what we want here. But I don't necessarily know about better approaches, silly repercussions, etc.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/33097382

Bugs closed: 506

Reviewed at https://rbcommons.com/s/twitter/r/925/