[TEST] remove global scope mention from ElasticsearchIntegrationTest#buildTestCluster

The global cluster gets created from a static block and shared through all tests in the same jvm. The `buildTestCluster` method can't get called passing in `Scope.GLOBAL`, hence removed its mention from it as it might be misleading. The only two scopes supported within the `buildTestCluster` method are `SUITE` and `TEST`.