Second – you can get and set the properties of the project and its elements. Like – overwriting project connection managers (I designed it with this particular need on my mind). You can now set the connection string the different server (or database) – in the PropertyPath of the PropertyCommand use
\Project\ConnectionManagers, write the name of the connection manager with the extension, and use one of the Properties. You can do it during the Test setup (or all tests setup), but not during the test suite setup, as ssisUnit is not aware of the project until it loads it into the memory.
Good on Bartosz for resurrecting a stable but moribund project and adding some enhancements.