BlockSet.fragment_by_block: add tests
BlockSet.fragment_by_block: add tests

Cover few cases in C++ version.

https://coveralls.io/builds/1896185/source?filename=src%2Fnpge%2Fcpp%2Fmodel.cpp

npge/lua-npge,starius/lua-npge,npge/lua-npge,starius/lua-npge,npge/lua-npge,starius/lua-npge