public FileReference addFile(String relativePath) {
        if (relativePath.isEmpty()) relativePath = "./";
        try {
            addFileInterface.addFile(Path.fromString(relativePath));
            FileReference fileReference = new FileReference(relativePath);
            entries.add(new Entry(relativePath, fileReference));
            return fileReference;
        } catch (IOException e) {
            throw new IllegalArgumentException(e);
        }
    }
A file registry for config server tests

@author hmusum
configserver/src/test/java/com/yahoo/vespa/config/server/filedistribution/MockFileRegistry.java