runtime: fix finalizer test on amd64

Not scanning the stack by frames means we are reintroducing
a few false positives into the collection. Run the finalizer registration
in its own goroutine so that stack is guaranteed to be out of
consideration in a later collection.

This is working around a regression from yesterday's tip, but
it's not a regression from Go 1.1.

R=golang-dev
TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/14290043