Modify poll interval setting in native realm test

This commit modifies the construction of the poll interval setting in
the native realm tests in response to upstream change
elastic/elasticsearchelastic/elasticsearch#2f638b5a23597967a98b1ced1deac91d64af5a44.

Original commit: elastic/x-pack-elasticsearch@c6f60f51f4

elasticsearch/x-pack/security/src/test/java/org/elasticsearch/integration/ClearRolesCacheTests.java

@@ -83,7 +83,7 @@ public class ClearRolesCacheTests extends NativeRealmIntegTestCase {
         logger.debug("using poller interval [{}]", pollerInterval);
         return Settings.builder()
                 .put(super.nodeSettings(nodeOrdinal))
-                .put(NativeRolesStore.POLL_INTERVAL_SETTING.getKey(), pollerInterval)
+                .put(NativeRolesStore.POLL_INTERVAL_SETTING.getKey(), pollerInterval.getStringRep())
                 .put(NetworkModule.HTTP_ENABLED.getKey(), true)
                 .build();
     }