run tests with warnings enabled

[graingert]

currenly there's a number of issues that have been opened due to deprecation warnings being ignored

the tests should be run with python -Werror runtests.py

[chrisdev]

@graingert I've noticed that pandas.utils.testing was deprecated. Utility testing mods now in pandas._testing. I've updated our test_manager so at least this silences one of the deprecation warnings.