[test] Create subdir in config dir if required (elastic#99218)

This commit updates the local cluster factory so that extra config files
(see `LocalSpecBuilder.configFile`) can be placed into subdirectories of
the config dir. This is needed when creating files for the
`FileSettingsService` - these k8s managed files are required to be in an
`"operator/"` directory within the general configuration directory.

test/test-clusters/src/main/java/org/elasticsearch/test/cluster/local/AbstractLocalClusterFactory.java

@@ -418,7 +418,18 @@ private void writeConfiguration() {
         }
 
         private void copyExtraConfigFiles() {
-            spec.getExtraConfigFiles().forEach((fileName, resource) -> resource.writeTo(configDir.resolve(fileName)));
+            spec.getExtraConfigFiles().forEach((fileName, resource) -> {
+                final Path target = configDir.resolve(fileName);
+                final Path directory = target.getParent();
+                if (Files.exists(directory) == false) {
+                    try {
+                        Files.createDirectories(directory);
+                    } catch (IOException e) {
+                        throw new UncheckedIOException(e);
+                    }
+                }
+                resource.writeTo(target);
+            });
         }
 
         private void createKeystore() {