Move BridgeUtilTests.

Tests in org.elasticsearch.entitlement.bridge are going to be uniquely hard to
test once we patch the bridge into java.base, due to Java's prohibition on
split packages.

Let's just move this guy to another package.

Author: prdoyle

libs/entitlement/src/test/java/org/elasticsearch/entitlement/BridgeUtilTests.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.entitlement;
+
+import org.elasticsearch.entitlement.bridge.Util;
+import org.elasticsearch.test.ESTestCase;
+
+import static org.elasticsearch.entitlement.BridgeUtilTests.MockSensitiveClass.mockSensitiveMethod;
+
+/**
+ * Note: this is not un the bridge package because that is a uniquely bad one to use for tests.
+ * Since:
+ * <ol>
+ *     <li>
+ *         we much patch the bridge module into {@code java.base} for it to be reachable
+ *         from JDK methods,
+ *     </li>
+ *     <li>
+ *         the bridge module exports {@code org.elasticsearch.entitlement.bridge}, and
+ *     </li>
+ *     <li>
+ *         Java forbids split packages
+ *     </li>
+ * </ol>
+ *
+ * ...therefore, we'll be unable to load any tests in the {@code org.elasticsearch.entitlement.bridge}
+ * package from the classpath.
+ * <p>
+ * Hence, we put this test in another package. It's still accessible during testing, though,
+ * because we export the bridge to `ALL-UNNAMED` anyway.
+ */
+public class BridgeUtilTests extends ESTestCase {
+
+    public void testCallerClass() {
+        assertEquals(BridgeUtilTests.class, mockSensitiveMethod());
+    }
+
+    /**
+     * A separate class so the stack walk can discern the sensitive method's own class
+     * from that of its caller.
+     */
+    static class MockSensitiveClass {
+        public static Class<?> mockSensitiveMethod() {
+            return Util.getCallerClass();
+        }
+    }
+
+}