def add_webdriver_subtest_suffix(test_name, subtest_name):
        """Appends a subtest name to a WebDriver test name. E.g.

        (test.py, foo.html) -> test.py>>foo.html
        (test.py, None)     -> test.py
        """
        if subtest_name:
            return test_name + Port.WEBDRIVER_SUBTEST_SEPARATOR + subtest_name
        return test_name