RFR: 8253285: Some java/util/StringJoiner tests do not explicitly specify required -XX:+CompactStrings

[Andrew Leonard]

Hi,
Please can I request a review of this test fix to explicitly specify 
-XX:+CompactStrings which is required for the tests in question, 
specifically for VMs (openj9) that do not default to +CompactStrings, see:
  PR  https://github.com/openjdk/jdk/pull/223
  bug https://bugs.openjdk.java.net/browse/JDK-8253285
Many thanks
Andrew

Andrew Leonard
Java Runtimes Development
IBM Hursley
IBM United Kingdom Ltd
internet email: andrew_m_leonard at uk.ibm.com 

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