Make sure user_db module is started if specified

This pull request makes sure (see tests) that `escalus:init_per_suite/1` starts `escalus_users`, and that `escalus:end_per_suite/1` stops `escalus_users`.

This lets any external modules implementing the `escalus_user_db` behavior initialize state and begin accepting user creation requests, and to later close down gracefully.

Also, `make ct` can now be run with an optional `SUITE=foo_bar_SUITE` parameter for faster feedback.