RFR(xs): 8186402: [TESTBUG] "Balance queues" output expected by test

[sangheon.kim]

Hi all,

Could I have some reviews for this tiny change of adding 
ParallelGCThreads=2 in TestPrintReferences.java?

The test is assuming always to have 'Balance queues:' log but it is only 
generated when the VM did queue balancing. Queue balancing happens when 
MT process is enabled(with +ParallelRefProcBalancingEnabled) or MT 
discovery is enabled. And MT discovery is enabled if we have multiple 
ParallelGCThreads (> 1).

Adding explicit thread setting of 'ParallelGCThreads=1' is easiest way 
to avoid this test failure on limited environments. i.e. machines which 
determine to use 1 ParallelGCThreads.

The test failure log also includes:
"[Java HotSpot(TM) 64-Bit Server VM warning: If the number of processors 
is expected to increase from one, then you should configure the number 
of parallel GC threads appropriately using -XX:ParallelGCThreads=N"

CR: https://bugs.openjdk.java.net/browse/JDK-8186402
Webrev: http://cr.openjdk.java.net/~sangheki/8186402/webrev.0/
Testing: JPRT

Thanks,
Sangheon
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