Fix tests on big endian machines

[QuLogic]

As in title:

• Force test arrays to be little-endian.
  These are saved on disk as little-endian and need to be compared as such.
• Add explicit byteorder on hexdigest tests.

TODO:

• Add unit tests and/or doctests in docstrings
• [N/A] Add docstrings and API docs for any new/modified user-facing classes and functions
• [N/A] New/modified features documented in docs/tutorial.rst
• [?] Changes documented in docs/release.rst
• [N/A] Docs build locally (e.g., run tox -e docs)
• AppVeyor and Travis CI passes
• Test coverage is 100% (Coveralls passes)

[jakirkham]

LGTM. Thoughts @zarr-developers/core-devs?

[jakirkham]

Thanks @QuLogic!

[jakirkham]

Have added a changelog entry for this in PR ( #441 ). If you could give that a quick look to make sure everything looks ok, that would be great 😄