test/run.go already executes tests in parallel where
possible. An unknown GOMAXPROCS value during the tests
is known to cause failures with tests that measure
allocations.

ref: https://groups.google.com/d/topic/golang-nuts/tgMhFJ3F5WY/discussion

R=fullung, minux.ma, r
CC=golang-dev
https://golang.org/cl/6847050