RFR: 8187831: Add stress test for clearing weak references

[Erik Helin]

Hi all,

I would like to add a new stress test that stresses clearing of weak 
references. From the comment in the test code:

// Tests that weak references are cleared. In this test case, the parent
// pointed from an instance of F to its TestFinalizer parent provokes an
// unusual situation, similar to the one found in ZipFile/Inflator.

Patch:
http://cr.openjdk.java.net/~ehelin/8187831/00/

Enhancement:
https://bugs.openjdk.java.net/browse/JDK-8187831

Testing:
- Run test successfully on Linux x86-64

Thanks,
Erik