Extend the testing framework with flexible constructors

[raphaelrobert]

Currently, it is very hard to maliciously manipulate structs that are defined in the spec. We need an easy way to set all fields in those structs to arbitrary values and to regenerate signatures and MACs (membership tag, confirmation tag).

[keks]

This seems to be addressed by the frankenstein framework. Should we close this and open individual issues for missing functionality?