TST skip `test_k_means_plus_plus_init_2_jobs` on Mac OS X 10.9. See GH-636
TST fix sklearn.ensemble.tests.test_bagging.test_parallel

n_jobs variable was unused previously, leaving n_jobs=-1 case untested for BaggingRegressor.
TST move Mac OS checking utility to sklearn.utils.testing
TST skip SparsePCA n_jobs!=1 test on Mac OS X because it hangs
TST remove n_jobs=-1 usages in tests
Merge pull request #2982 from kmike/fix-macos-hangs

[MRG+1] MacOS X testing improvements