This is what we do:
In general, you don't need to deal with scoping your information to tenants in your unit tests. So just write these tests as if there was no scoping going on.
The only exception to this is if you want to specifically test something related to the scoping. We do this very rarely. Acts_as_tenant has its own test suite to test the scoping (but then I would say that, since I wrote it ;).
If you must deal with AaT in your unit tests you can set the current_tenant directly:
ActsAsTenant.current_tenant = Account.first
It's then important that you clean up that tenant in an after filter (set it to nil), or you will be in debug hell.