The current design for Fair Queueing for Server Requests has a
problem: if the min-max fair result stays different from an even
division for a long time and no queue involved in the imbalance goes
empty then the imbalance keeps accruing in queue virtual state times.
This commit adds a test that demonstrates the problem.
It also has some other tweaks to make other tests less flaky.
Factor the big scenario-testing func into pieces, with supporting
structs.
f3fdd5cf9f