Respect DB_IP and DB_PORT in mongodb tests

It is usually preferred to run tests on a test-local database with dedicated mongod instance rather than using the system-wide database. In order to support that, an easy way of providing the proper port is handy.

For example, pymongo test suite uses `DB_IP` and `DB_PORT` environment variables to allow providing an alternate mongod instance. Using the same variable names in IPython's test suite seems like a good idea.