RFR 7112404: 2 tests in java/lang/management/ManagementFactory fails with G1 because expect non-zero pools

[Jaroslav Bachorik]

Please, review this simple test fix.

Issue:  https://bugs.openjdk.java.net/browse/JDK-7112404
Webrev: http://cr.openjdk.java.net/~jbachorik/7112404/webrev.00

The tests assume the MemoryUsage#commited values to be positive (>0) 
while according to the MemoryUsage only negative values are invalid. 
Therefore the tests should check and fail only when MemoryUsage#commited 
is < 0.

Thanks,

-JB-