Can't test non-static return values

[devenjarvis]

Right now sundew expects you to be able to assert that a function returns a specific value. Sometimes this isn't feasible, such as when testing a function that returns a generated id. Normally we'd test properties of the return like "A value was returned, that value is a string, it shoul be shaped like: XXX-XXXX-XXXXXXXXX, etc". Sundew should be extended to support more expressive return assertions if we want it to be useful across all testing circumstances