Fixup engine test deps.

I was starting to look at coveralls and ran into some missing deps on my chosen victim test target, this fixes the deps up.

Testing Done:
```console
$ PANTS_PY_COVERAGE=1 ./pants goal test tests/python/pants_test/engine
$ PANTS_PY_COVERAGE=1 ./pants goal test tests/python/pants_test/engine:test_engine
$ PANTS_PY_COVERAGE=1 ./pants goal test tests/python/pants_test/engine:test_round_engine
$ COVERALLS_REPO_TOKEN=XYZ coveralls
```

https://coveralls.io/jobs/2668487

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/34040483

Bugs closed: 369, 540

Reviewed at https://rbcommons.com/s/twitter/r/968/