@Test
  public void testAllowFallbackToSimple() throws Exception {
    clientConf.setBoolean(RpcClient.IPC_CLIENT_FALLBACK_TO_SIMPLE_AUTH_ALLOWED_KEY, true);
    rpcClient = createClient();
    assertEquals("allow", echo("allow"));
  }
Test secure client connecting to a non secure server, where we have multiple server principal
candidates for a rpc service. See HBASE-28321.
hbase-server/src/test/java/org/apache/hadoop/hbase/security/TestMultipleServerPrincipalsFallbackToSimple.java