runtime: fix stack memory test

runtime: fix stack memory test

Stand-alone this test is fine.  Run together with
others, however, the stack used can actually go
negative because other tests are freeing stack
during its execution.

This behavior is new with the new stack allocator.
The old allocator never returned (min-sized) stacks.

This test is fairly poor - it needs to run in
isolation to be accurate.  Maybe we should delete it.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/119330044