[OpenJDK 2D-Dev] RFR: 8145808: [PIT] test java/awt/Graphics2D/MTGraphicsAccessTest

[Phil Race]

https://bugs.openjdk.java.net/browse/JDK-8145808
http://cr.openjdk.java.net/~prr/8145808/

I believe we have a test bug where the decrement on the
counter is being treated as atomic but it is carried out on
different threads protected only by 'volatile'.

This fix does seem likely to make the test more reliable.
Note that "numexceptions" technically has the same issue
but it does not matter as it is mainly informational but
and any number > 0 is all that ever matters.
So I left that alone.

-phil.