Fix RealmInteg test failures

As part of the changes in elastic#31234,the password verification logic
determines the algorithm used for hashing the password from the
format of the stored password hash itself. Thus, it is generally
possible to validate a password even if it's associated stored hash
was not created with the same algorithm than the one currently set
in the settings.
At the same time, we introduced a check for incoming client change
password requests to make sure that the request's password is hashed
with the same algorithm that is configured to be used in the node
settings.
In the spirit of randomizing the algorithms used, the
{@code SecurityClient} used in the {@code NativeRealmIntegTests} and
{@code ReservedRealmIntegTests} would send all requests dealing with
user passwords by randomly selecting a hashing algorithm each time.
This meant that some change password requests were using a different
password hashing algorithm than the one used for the node and the
request would fail.
This commit changes this behavior in the two aforementioned Integ
tests to use the same password hashing algorithm for the node and the
clients, no matter what the request is.

Resolves elastic#31670

x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/authc/esnative/ReservedRealmIntegTests.java

@@ -8,13 +8,16 @@
 import org.elasticsearch.ElasticsearchSecurityException;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthResponse;
 import org.elasticsearch.common.settings.SecureString;
+import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.test.NativeRealmIntegTestCase;
 import org.elasticsearch.xpack.core.security.action.user.ChangePasswordResponse;
+import org.elasticsearch.xpack.core.security.authc.support.Hasher;
 import org.elasticsearch.xpack.core.security.client.SecurityClient;
 import org.elasticsearch.xpack.core.security.user.BeatsSystemUser;
 import org.elasticsearch.xpack.core.security.user.ElasticUser;
 import org.elasticsearch.xpack.core.security.user.KibanaUser;
 import org.elasticsearch.xpack.core.security.user.LogstashSystemUser;
+import org.junit.BeforeClass;
 
 import java.util.Arrays;
 
@@ -29,6 +32,22 @@
  */
 public class ReservedRealmIntegTests extends NativeRealmIntegTestCase {
 
+    private static Hasher hasher;
+
+    @BeforeClass
+    public static void setHasher() {
+        hasher = getFastStoredHashAlgoForTests();
+    }
+
+    @Override
+    public Settings nodeSettings(int nodeOrdinal) {
+        Settings settings = Settings.builder()
+            .put(super.nodeSettings(nodeOrdinal))
+            .put("xpack.security.authc.password_hashing.algorithm", hasher.name())
+            .build();
+        return settings;
+    }
+
     public void testAuthenticate() {
         for (String username : Arrays.asList(ElasticUser.NAME, KibanaUser.NAME, LogstashSystemUser.NAME, BeatsSystemUser.NAME)) {
             ClusterHealthResponse response = client()
@@ -76,7 +95,7 @@ public void testChangingPassword() {
         }
 
         ChangePasswordResponse response = securityClient()
-            .prepareChangePassword(username, Arrays.copyOf(newPassword, newPassword.length), getFastStoredHashAlgoForTests())
+            .prepareChangePassword(username, Arrays.copyOf(newPassword, newPassword.length), hasher)
                 .get();
         assertThat(response, notNullValue());