Pass allow security manager flag in gradle test policy setup plugin (#111725)

The `java.security.manager=allow` system property is required when
running tests on newer Java versions with the security manager
deprecated. As such, it should be set in our
`GradleTestPolicySetupPlugin` so that it's done for external plugin
authors.

build-tools/src/main/java/org/elasticsearch/gradle/test/GradleTestPolicySetupPlugin.java

@@ -22,6 +22,9 @@ public class GradleTestPolicySetupPlugin implements Plugin<Project> {
             test.systemProperty("tests.gradle", true);
             test.systemProperty("tests.task", test.getPath());
 
+            // Flag is required for later Java versions since our tests use a custom security manager
+            test.jvmArgs("-Djava.security.manager=allow");
+
             SystemPropertyCommandLineArgumentProvider nonInputProperties = new SystemPropertyCommandLineArgumentProvider();
             // don't track these as inputs since they contain absolute paths and break cache relocatability
             nonInputProperties.systemProperty("gradle.dist.lib", gradle.getGradleHomeDir().getAbsolutePath() + "/lib");