Multitests for tgrade-voting-contract

[hashedone]

tg-voting-contract is not a contract itself, but utility commonizing other voting contracts stuff, but it should be actually tested in muiltitests because of its dependence on another contract (in particular group contract).

Tests may be based on tgrade-oc-proposals tests, but should be done in our "Suite" pattern. To make such tests, the mock/stub contract just for testing purposes should be created on multitests module of tg-voting-contract.

[ethanfrey]

Let's do this for v0.5.3