Improve define/declare mixin tests

[iboB]

Currently most of the tests of object manipulation from mixins created using DYNAMIX_DECLARE_MIXIN and DYNAMIX_DEFINE_MIXIN are in t-define_mixin.cpp (and some in t-msg.cpp). It's too dense and likely there are use cases which are missed. Split them in more files using a test library with mixins, rules, and the like.