adapt SchedulerSpec rate test to 10ms time slices, see #3391

using a larger sample will also make the spread of the resulting
calculated rate smaller; the referenced ticket is about a deviation from
the target rate which was slightly out of bounds
Merge pull request #1508 from akka/wip-3391-SchedulerSpec-∂π

adapt SchedulerSpec rate test to 10ms time slices, see #3391