[TESTBUG] a few StringTable tests should use new -XX:+VerifyStringTableAtExit option

A new -XX:+VerifyStringTableAtExit option is being added by the fix for
the following bug:

    JDK-8019835 Strings interned in different threads equal but does not ==

It would be good to add the new option to a few of the StringTable
stress tests so that we get more test coverage.

In the hunt for JDK-8019835, I found the following VM/NSK test to be useful:

    gc/gctests/StringInternSync

And that's the test I used for developing the new StringTable verification code.