runtime: benchmark for ping-pong in the presence of a CPU hog

This benchmark demonstrates a current problem with the scheduler where
a set of frequently communicating goroutines get very little CPU time
in the presence of another goroutine that hogs that CPU, even if one
of those communicating goroutines is always runnable.

Currently it takes about 0.5 milliseconds to switch between
ping-ponging goroutines in the presence of a CPU hog:

BenchmarkPingPongHog	    2000	    684287 ns/op

Change-Id: I278848c84f778de32344921ae8a4a8056e4898b0
Reviewed-on: https://go-review.googlesource.com/9288


Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>