Further improvements of network package tests

[ivanovpetr]

The network package now have a unit tests. They work well but there are still a lot things to improve. Two main topics I see important are:

1. Moving all utilities related to stubbing low level stuff (genesis, gentx, cosmosclient, responses) to more global scope.
2. Improving predefined fixtures in order to reduce amount of code used for setup.

Also @Pantani mentioned that cosmos-sdk uses gomock package for unit tests. We can explore possible benefits of this library over testify/mock