Revision 1701816

Backport:
LUCENE-6760: Prevent test failure in Java 9 b76+
SOLR-7958: Move TestUtil#randomWhitespace to the only Solr test that is using it
LUCENE-6760: if the list of whitspace characters changes again in a future JVM, fail fast with a clear error that the test needs corrected -- even if jvm assertions are disabled OR we get really lucky with the chars radnomly selected (merge r1698227)