testing: allow benchmarks to print and fail

testing: allow benchmarks to print and fail
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