Have `distarray/core` tests pass which catching `NullCommError`...

instead of raising a `unittest.SkipTest`.
Have the `nprand` tests `pass` when catching a `NullCommError`...

and have `init_base_comm` raise a `NullCommError` instead of a `MPICommError` for consistency with `BaseLocalArray.__init__`'s behavior.
Have basic DAP tests pass on `NullCommError`.
Run `distarray/core/tests` with 12 MPI processes.
Merge pull request #62 from enthought/bugfix/unnecessary-test-skips

Don't skip tests unnecessarily