Add tests of learned agglomeration

Two tests are currently skipped because of a problem with the random
forest training -- setting the random state does not eliminate
differences.

@DocSavage, I've shared a gist showing how I generated the test data, including the two tests that are currently being skipped. I can't figure out why the training datasets come out different between [example-run.py](https://gist.github.com/jni/7215110) (the gist) and `test_gala.py` (created in this PR), since I set the random state... Perhaps some non-numpy random number generator is used... Anyway, for now those tests are skipped.