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

test/jdk/java/util/StringJoiner/MergeTest
test/jdk/java/util/StringJoiner/StringJoinerTest
do not explicitly specify required -XX:+CompactStrings, as these tests perform MAX_ARRAY_LENGTH large String array computations, which without CompactStrings will fail.
eg.
14:09:02 test MergeTest.OOM(): failure
14:09:02 java.lang.OutOfMemoryError: UTF16 String size is 2147483639, should be less than 1073741823
14:09:02 at java.base/java.lang.StringUTF16.newBytesFor(StringUTF16.java:50)
14:09:02 at java.base/java.lang.String.repeat(String.java:3967)
14:09:02 at MergeTest.OOM(MergeTest.java:175)

Hotspot defaults to +CompactStrings, however not every VM necessarily does, for example Eclipse OpenJ9 defaults to -CompactStrings.