RFR: JDK-8249258 java/util/StringJoiner/StringJoinerTest.java failed due to OOM (JDK 15)

[Jim Laskey]

The test was failing on a newly added mac mini. I've reduced the memory stress by introducing a shared maximum sized string. I don't recommend reducing the 4G memory requirement -- multiple new large objects are constructed in the tests hoping to create an OOM.

JBS: https://bugs.openjdk.java.net/browse/JDK-8249258 <https://bugs.openjdk.java.net/browse/JDK-8249258>
webrev: http://cr.openjdk.java.net/~jlaskey/8249258/webrev-01 <http://cr.openjdk.java.net/~jlaskey/8249258/webrev-01>