Added solver callback for testing nets

[ChWick]

The Solvers::Callback received methods for communication during the
testing of a net:

• on_test_start(int id): Testing of the net with id started
• on test_end(int id): Testing of the net with id ended
• on_test_iter_start(int id, int iter): Start of the iter'th iteration

I need this functions to be able to dynamically write into input blobs during testing in C++.
Currently I only can generate data for the training stage using the 'on_start' callback.