Test: Use --illegal-access=warn for tests (#28437)

This commit conditionally adds the --illegal-access=warn flag when tests
are run with java 9. Currently, testing on java 9 triggers a warning
about illegal access from mockito. While that should be fixed (by
updating to a newer mockito base for securemock), the stderr warning we
get is only the first one. Thankfully that is the only one, but this
change will enable finding all such illegal accesses in the future.

buildSrc/src/main/groovy/org/elasticsearch/gradle/BuildPlugin.groovy

@@ -567,6 +567,9 @@ class BuildPlugin implements Plugin<Project> {
             File heapdumpDir = new File(project.buildDir, 'heapdump')
             heapdumpDir.mkdirs()
             jvmArg '-XX:HeapDumpPath=' + heapdumpDir
+            if (project.runtimeJavaVersion >= JavaVersion.VERSION_1_9) {
+                jvmArg '--illegal-access=warn'
+            }
             argLine System.getProperty('tests.jvm.argline')
 
             // we use './temp' since this is per JVM and tests are forbidden from writing to CWD