add lender system tests pool closing with juniorTokenPrice zero

[xmxanuel]

In general if junior starts to lose its value the maxDROPRatio constraint will be violated. Before the juniorAsset value approaches zero, DROP holders could still leave the pool because DROP redeem action have highest priority and it improves the DROPRatio.

If juniorAsset == 0, the current DROP ratio would be ONE and could maybe block DROP redeems.

Write more systems tests with juniorTokenPrice zero to identify potential problems