Skip transport client plugin installed on JDK 9

This commit adds an assumption to
PreBuiltTransportClientTests#testPluginInstalled on JDK 9. The
underlying issue is that Netty attempts to access sun.nio.ch but this
package is not exported from java.base on JDK 9. This throws an uncaught
InaccessibleObjectException causing the test to fail. This assumption
can be removed when Netty 4.1.6 is released as it will include a fix for
this scenario.

Relates #20251

client/transport/src/test/java/org/elasticsearch/transport/client/PreBuiltTransportClientTests.java

@@ -20,6 +20,7 @@
 package org.elasticsearch.transport.client;
 
 import com.carrotsearch.randomizedtesting.RandomizedTest;
+import org.apache.lucene.util.Constants;
 import org.elasticsearch.client.transport.TransportClient;
 import org.elasticsearch.common.network.NetworkModule;
 import org.elasticsearch.common.settings.Settings;
@@ -40,6 +41,8 @@ public class PreBuiltTransportClientTests extends RandomizedTest {
 
     @Test
     public void testPluginInstalled() {
+        // TODO: remove when Netty 4.1.5 is upgraded to Netty 4.1.6 including https://github.com/netty/netty/pull/5778
+        assumeFalse(Constants.JRE_IS_MINIMUM_JAVA9);
         try (TransportClient client = new PreBuiltTransportClient(Settings.EMPTY)) {
             Settings settings = client.settings();
             assertEquals(Netty4Plugin.NETTY_TRANSPORT_NAME, NetworkModule.HTTP_DEFAULT_TYPE_SETTING.get(settings));
@@ -49,9 +52,7 @@ public class PreBuiltTransportClientTests extends RandomizedTest {
 
     @Test
     public void testInstallPluginTwice() {
-
-        for (Class<? extends Plugin> plugin : Arrays.asList(ReindexPlugin.class, PercolatorPlugin.class,
-            MustachePlugin.class)) {
+        for (Class<? extends Plugin> plugin : Arrays.asList(ReindexPlugin.class, PercolatorPlugin.class, MustachePlugin.class)) {
             try {
                 new PreBuiltTransportClient(Settings.EMPTY, plugin);
                 fail("exception expected");