Currently the property test.cds.copy.child.stdout is set to "false" by default. This property controls how the output of Child Processes used in CDS testing is saved. The output produced by the child process is important for investigating test failures.
When property is false, the output is saved into the logfile into the JT-Reg's current directory. If the property is set to true, the output is also copied into the output of the (main) test process, hence it is saved into the JTR file.

Unfortunately currently the log file (LogFileName.stdout) is not retained by the automated test system. Until this is fixed this property should be set to true by default. Note, this is a regression introduced by "JDK-8177728 - [TESTBUG] Improve CDS test utils".