cmd/coordinator: run benchmarks on try work

Benchmarks are treated as unit tests and distributed to the test
helpers, which allows them to fit in our 5m trybot budget.

Currently we only run the go1 and x/benchmarks. Running package
benchmarks is a TODO.

This feature is disabled by default, and is enabled by the
"farmer-run-bench" project attribute.

Updates golang/go#19178
Updates golang/go#19871

Change-Id: I9c3a14da60c3662e7e2cb4e71953060915cc4364
Reviewed-on: https://go-review.googlesource.com/38306
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
8 files changed