Fixes #4008.

Run a background goroutine that wastes CPU to trick the
power management into raising the CPU frequency which,
by side effect, makes sleep more accurate on arm.

=== RUN TestParallelSleep
--- PASS: TestParallelSleep (1.30 seconds)
_cgo_gotypes.go:772:    sleep(1) slept for 1.000458s

R=minux.ma, r
CC=golang-dev
https://golang.org/cl/6498060