test: fix bad assumption in pummel/test-vm-memleak

pummel/test-vm-memleak is an old test that assumes the fairly
aggressive heuristics that were common with the old collector.

The current garbage collector has a more laissez-faire attitude.
Put an upper limit on the size of the old space and update the
test's expectations.

PR-URL: https://github.com/iojs/io.js/pull/280


Reviewed-By: Bert Belder <bertbelder@gmail.com>