RFR(XS): 8224165: [TESTBUG] Docker tests produce excessive output

[mikhailo.seledtsov at oracle.com]

Please review this change that limits how much of child process output 
is copied into
the main test output. Docker tests child processes produce too much 
output at times, which results in JT harness
trimming the output, and can lead to loss of important diagnostic 
information.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8224165
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8224165.00/
     Testing:
        Ran HotSpot and JDK docker tests
            test/hotspot/jtreg/containers/docker/
            test/jdk/jdk/internal/platform/docker/

        No new failures

Thank you,
Misha