Add test command and fix depends

The tryton test API has changed and now expects the
configuration to be preset and DB_NAME to be there in
the environment variables.

The suite makes it convenient to run tests using
`python setup.py test` and `python setup.py test_on_postgres`

See tryton/trytond@95b51d3893253414e4e318ed49f056cbefbcd7da
Install nereid-test from repo