10 RFR: 8169517: WhiteBox should provide concurrent GC phase control

[Kim Barrett]

> On Apr 13, 2017, at 8:01 AM, Dmitry Fazunenko <dmitry.fazunenko at oracle.com> wrote:
> 
> Kim,
> 
> new version looks good to me.
> Thank you very much for addressing my concerns.
> 
> — Dima

Thanks for prodding me to improve the tests.