Lucene with Java 1.9 doesn't support unmap in MMapDirectory until Luc…

…ene 6.4.0 (#22501)

The Java APIs for unmapping are changing, and Lucene 6.4.0 will
support it correctly, so until then we need an assume in this test
case.

See https://issues.apache.org/jira/browse/LUCENE-6989 for details.

Closes #22496

core/src/test/java/org/elasticsearch/common/lucene/LuceneTests.java

@@ -43,6 +43,7 @@
 import org.apache.lucene.store.MMapDirectory;
 import org.apache.lucene.store.MockDirectoryWrapper;
 import org.apache.lucene.util.Bits;
+import org.apache.lucene.util.Constants;
 import org.elasticsearch.test.ESTestCase;
 
 import java.io.IOException;
@@ -405,6 +406,7 @@ public void testAsSequentialAccessBits() throws Exception {
      */
     public void testMMapHackSupported() throws Exception {
         // add assume's here if needed for certain platforms, but we should know if it does not work.
+        assumeTrue("Lucene 6.3.0 doesn't support unmap with Java 9", Constants.JRE_IS_MINIMUM_JAVA9 == false);
         assertTrue("MMapDirectory does not support unmapping: " + MMapDirectory.UNMAP_NOT_SUPPORTED_REASON, MMapDirectory.UNMAP_SUPPORTED);
     }
 }