Drop support for `setup.py test`

Drop support for running tests via setup.py test, that is considered deprecated. tox is our primary frontend for running tests, so this change ensures that we don't have to support multiple ways of running tests.

For testing against a specific installation environment, tox is not quite practical. But in these cases, one can run pytest directly. So, there is no need for this additional indirection.

This was discussed in the following mailing list thread: https://mail.gnome.org/archives/buildstream-list/2019-December/msg00006.html.