ATokenFlexVoting: Test that borrowers can vote and that votes aren't double counted

[davidlaprade]

We need to make sure that liquidity is taken into consideration when casting votes from aTokens. If some of the governance tokens have been borrowed, this should reduce the amount that the aToken can vote with. We don't want to risk double counting votes!

This was a concern of some folks from Aave, per telegram chat, so I've added some additional test assertions to check for it