cmd/bench: wait for load average to drop before starting

The coordinator runs tests on a freshly booted VM which may still be
running background boot tasks when bench starts.

For minimal noise, wait for the system load average to drop (indicating
background tasks have completed) before continuing with benchmarking.

For golang/go#49207.

Change-Id: I8df01592fea31d49eae54074213e202b21d5728a
Reviewed-by: Michael Knyszek <>
Trust: Michael Pratt <>
Run-TryBot: Michael Pratt <>
TryBot-Result: Gopher Robot <>
3 files changed