[ftr] update svl shared config with cluster settings (elastic#196336)

## Summary

Adding Elasticsearch cluster settings to replicate MKI cluster setup for
FTR.

x-pack/test_serverless/shared/config.base.ts

@@ -95,6 +95,10 @@ export default async () => {
         `xpack.security.authc.realms.jwt.jwt1.pkc_jwkset_path=${getDockerFileMountPath(jwksPath)}`,
         `xpack.security.authc.realms.jwt.jwt1.token_type=access_token`,
         'serverless.indices.validate_dot_prefixes=true',
+        // controller cluster-settings
+        `cluster.service.slow_task_logging_threshold=15s`,
+        `cluster.service.slow_task_thread_dump_timeout=5s`,
+        `serverless.search.enable_replicas_for_instant_failover=true`,
       ],
       ssl: true, // SSL is required for SAML realm
     },