Refactors the benchmarks and test code.
Now benchmarks can call Errorf, Fail, etc.,
and the runner will act accordingly.

Because functionality has been folded into an
embedded type, a number of methods' docs
no longer appear in godoc output. A fix is
underway; if it doesn't happen fast enough,
I'll add wrapper methods to restore the
documentation.

R=bradfitz, adg, rsc
CC=golang-dev
https://golang.org/cl/5492060