We generate certificates that are intended to be used for server authentication in a couple of places:

- `bin/elasticsearch-certutil http` command  ( See `CertGenUtils.generateSignedCertificate` in `HttpCertificateCommand` )
- TLS auto-configuraion for node startup ( see [`AutoConfigureNode`](https://github.com/elastic/elasticsearch/blob/814f7f9f52cb8c05db6465b9bfc818ea5d76c575/x-pack/plugin/security/cli/src/main/java/org/elasticsearch/xpack/security/cli/AutoConfigureNode.java#L454) )

In these cases we know that the certificate is meant to be used for server authentication, so we can set Extended Key Usage extension to `1.3.6.1.5.5.7.3.1`
x-pack/plugin/security/cli/src/test/java/org/elasticsearch/xpack/security/cli/CertGenUtilsTests.java
import java.net.InetAddress;
import java.security.KeyPair;
import java.security.KeyStore;
import java.security.cert.X509Certificate;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import javax.net.ssl.TrustManager;
import javax.net.ssl.TrustManagerFactory;
import javax.net.ssl.X509ExtendedTrustManager;
import javax.security.auth.x500.X500Principal;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.is;
import static org.hamcrest.Matchers.notNullValue;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.verifyNoMoreInteractions;
import static org.mockito.Mockito.when;
/**
 * Unit tests for cert utils
 */
public class CertGenUtilsTests extends ESTestCase {
    @BeforeClass
    public static void muteInFips() {
        assumeFalse("Can't run in a FIPS JVM", inFipsJvm());
    }
    public void testSerialNotRepeated() {
        int iterations = scaledRandomIntBetween(10, 100);