Previously we simply create a lot of the target objects and check
if the process crash due to OOM. Due to how we use emphemeron GC
to handle memory management, which is inefficient but necessary
for correctness, the tests can produce false positives as
the GC isn't efficient enough to catch up with a very fast
heap growth.

This patch uses a new checkIfCollectable() utility to terminate the
test early once we detect that any of the target object can actually
be garbage collected. This should lower the chance of false positives.
As a drive-by this also allows us to use setImmediate() to grow the
heap even faster to make the tests run faster.