public File mkdir(String dirName) {
        File file = new File(tempFolder(), dirName);
        file.mkdirs();
        createdFiles.add(file);
        return file;
    }
@Test
    public void shouldCreateDirsInTempDirectory() throws IOException {
        File dir = files.mkdir("foo");
        File parentFile = dir.getParentFile();
        assertThat(parentFile.getName(), is("cruise"));
        assertThat(parentFile.getParentFile(), is(tmpDir()));
    }