implement default caller value inside test_spice

I took a look at the `test_spice` code as discussed in issue https://github.com/duckduckgo/duckduckgo/issues/39, and due to the current structure it does not seem possible to simply add a default for the `caller` as that information is out of scope inside `test_spice`. That said, I have implemented a default value for the `call_type`.