HSEARCH-1728 introduce new `metadata_writes_async` configuration property for Infinispan

 - https://hibernate.atlassian.net/browse/HSEARCH-1728

a lot of hoops to test what you are after and quite fragile as well. Why don't you use a `@BMRule` and verify that `org.infinispan.lucene.directory.BuildContext#writeFileListAsynchronously gets called accordingly. The test becomes much easier imo.