include tests in distribution package tarball

include tests in distribution package tarball

Prior to this change, the "remoto/tests/" directory was present in Git,
but not present in the release tarballs we shipped on PyPI.

Discussed this with Travis and Alfredo in #ceph-devel today. Alfredo
pointed out that the reason the remoto tarball does not include the
tests directory is that there is no __init__.py file there for distutils
to find.

The solution Alfredo recommends is to add the directory to MANIFEST.in.
With this change, the tests directory is present in the source package
tarball.

The purpose of including the tests in the distribution is to 1) be
consistent with what we're doing with the ceph-deploy package, and 2)
make the tests easily available to run during the RPM builds.

Signed-off-by: Ken Dreyer <kdreyer@redhat.com>