raft: randomized_nemesis_test: tick virtual clock less aggressively
raft: randomized_nemesis_test: tick virtual clock less aggressively

Currently each tick of the virtual clock immediately schedules the next one
at the end of the task queue, but this is too aggressive. If a tick
generates work that need two tasks to be scheduled one after another
such implementation will make the task queue grow to infinity. Considering
that in the debug mode even ready future causes preemption and task
queue shuffling may cause two or more ticks to be executed without any
other work done in the middle it is very easy to get to such situation.

The patch changes the virtual clock to tick only when a shard is idle.
Message-Id: <63f07fd24af1c363865dc48de0c2853971ff1abc@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla