[test] Treasury surplus burns in absence of proposals

[ekovalev]

A test that demonstrates that in case no proposals have been registered with Treasury, the treasury account positive balance is halved every SpendPeriod blocks (1 day worth of blocks in Vara runtime) thereby causing the total issuance reduction. Unless specifically handled - that is, a BurnDestination is provided that takes care of the resulting NegativeImbalance.
Apparently, the rationale behind this is that Treasury is not supposed to store funds, effectively reducing the amount in circulation. It's only when a proposal is created, funds can flow in the Treasury until enough thereof is accumulated.

It means that if the idea is to be able to store funds in Treasury for some future purpose, we need to funnel the burned amount back to the Treasury itself.