RFR: 8187831: Add stress test for clearing weak references

[Kim Barrett]

> On Sep 22, 2017, at 2:54 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> Sorry, but I found the bug description and the test description pretty
> much indecipherable.  I had to study the code to have any guess as to
> what is being tested here.

In particular, I found the use of “parent” in the description confusing, since I
see nothing in the code that looks like a parental relationship.

Also, do we actually need this test at all?  Recall that finalize was deprecated in jdk9.
We should be removing uses of finalize and associated tests.  And in the specific case
of ZipFile, someone is already working on that.