As you discovered, this isn't currently possible as the mocking procedures don't accept three part names. This is something that's been covered at the User feedback forums of SQL Test (RedGate's product that acts as a front end to tSQLt) at : http://sqltest.uservoice.com/forums/140716-sql-test-forum/suggestions/2421628-reduce-the-footprint
Dennis Lloyd, one of the authors of the tSQLt framework wrote towards the end of that thread that support of a separate 'tSQLt' database was something they would keep under consideration.
Also a related issue of mocking remote objects at http://sqltest.uservoice.com/forums/140716-sql-test-forum/suggestions/2423449-being-able-to-mock-fake-remote-objects
I hope that helps,
Dave