runtime: disable parallel for tests under race detector.
The race detector does not understand ParFor synchronization, because it's implemented in C.
If run with -cpu=2 currently race detector says:
WARNING: DATA RACE
Read by goroutine 5:
runtime_test.TestParForParallel()
src/pkg/runtime/parfor_test.go:118 +0x2e0
testing.tRunner()
src/pkg/testing/testing.go:301 +0x8f
Previous write by goroutine 6:
runtime_test.func·024()
src/pkg/runtime/parfor_test.go:111 +0x52
R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/
6811082