RFR(XS): JDK-8180393: [TESTBUG] CDSTestUtils property test.cds.copy.child.stdout should be true by default

[mikhailo]

Please review this trivial change; see bug description for rational and 
details.

JBS: https://bugs.openjdk.java.net/browse/JDK-8180393
Webrev: http://cr.openjdk.java.net/~mseledtsov/8180393.00/
Testing:
         1. Locally: exercised all CDS tests locally on Linux-x64

             Checked several jtr files to make sure that stdout of child 
process is copied into the jtr file

             PASS

         2. Automated: CDS tests on multiple platforms via automated 
test system

             In progress


Thank you,
Misha