runtime: move TestGcSys into a separate process
Fixes issue 4904.
The problem was that when the test runs the heap had grown to ~100MB,
so GC allows it to grow to 200MB, and so the test fails.
Moving the test to a separate process makes it much more isolated and stable.

R=golang-dev, minux.ma
CC=golang-dev
https://codereview.appspot.com/7441046
runtime: deadlock tests now work with GOMAXPROCS>1
Fixes issue 4826.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7434046