Run validation on the generated WDL output using wdltool.

def _validate(wdl_file):
    """Run validation on the generated WDL output using wdltool.
    """
    start_dir = os.getcwd()
    os.chdir(os.path.dirname(wdl_file))
    print("Validating", wdl_file)
    subprocess.check_call(["wdltool", "validate", wdl_file])
    os.chdir(start_dir)