While investigating failures in test/java/util/zip/ZipFile/Collectible.java,
I discovered some robustness improvements to the test.

Instead of

public static int births = 0;
public static int deaths = 0;

we should be using Atomics

For added chance of running finalizers, we should call System.runFinalization twice.

The garbage-generating code should be moved into a separate method, to help
guarantee unreachability.