starting lots of workers/schedulers in tests starts a lot of _measure_tick PeriodicCallbacks

`_measure_tick` runs by default every 20ms for every `distributed.core.Server` created, eg every `Worker` and `Scheduler`

https://github.com/dask/distributed/blob/39c5e8856cb05ad1833a0f6ee6c3fea0ddab1c1c/distributed/core.py#L348-L362

this can be a particular problem in tests such as `test_adapt_quickly` that starts 10 workers and a scheduler and so has 11 of these calls every 20ms