Add tests to sdist.

In order to validate installation it's very convenient to have the tests
as part of the source distribution.  This greatly assists native
packaging that might occur (i.e. Gentoo ebuilds).

Signed-off-by: Alex Brandt <alunduil@alunduil.com>
Merge pull request #585 from alunduil/add-tests-to-sdist

Add tests to sdist.