Add tests for SystemSubject

Signed-off-by: Craig Perkins <cwperx@amazon.com>

Author: cwperks

server/src/test/java/org/opensearch/identity/SystemSubjectTests.java

@@ -0,0 +1,28 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.identity;
+
+import org.opensearch.test.OpenSearchTestCase;
+import org.opensearch.threadpool.TestThreadPool;
+import org.opensearch.threadpool.ThreadPool;
+
+public class SystemSubjectTests extends OpenSearchTestCase {
+    public void testSystemSubject() {
+        ThreadPool threadPool = new TestThreadPool(getTestName());
+        SystemSubject systemSubject = SystemSubject.getInstance();
+        assertEquals("system", systemSubject.getPrincipal().getName());
+        systemSubject.initialize(threadPool);
+        assertFalse(threadPool.getThreadContext().isSystemContext());
+        systemSubject.runAs(() -> {
+            assertTrue(threadPool.getThreadContext().isSystemContext());
+            return false;
+        });
+        terminate(threadPool);
+    }
+}