runtime: fix deadlock in parallel for test

runtime: fix deadlock in parallel for test
The deadlock occurs when another goroutine requests GC
during the test. When wait=true the test expects physical parallelism,
that is, that P goroutines are all active at the same time.
If GC is requested, then part of the goroutines are not scheduled,
so other goroutines deadlock.
With wait=false, goroutines finish parallel for w/o waiting for all
other goroutines.
Fixes #3954.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6820098