@NotNull
  private Path copyResource(URL resource, Path file) throws IOException {
    try (InputStream inputStream = resource.openStream()) {
      Files.copy(inputStream, file);
    }
    return file;
  }
Basically the same as {@link DirectoryFixture#dir(String, Closure)}, but with the current root as base.
spock-core/src/main/java/spock/util/io/FileSystemFixture.java