input: Write a function named `automate_login` that automates the login process on a given website using Selenium. The function should take the target website URL as an input parameter and return a string indicating the success of the login process. If the login is successful, the function should print "Login successful!" and return "Done". Use the following assert statements to test the function:
  assert automate_login('http://www.example.com') == "Done"
  assert automate_login('http://www.anothersite.com') == "Done"
output: def automate_login(url):
      driver = webdriver.Chrome()
      driver.get(url)
      username = driver.find_element_by_id('username')
      password = driver.find_element_by_id('password')
      login_button = driver.find_element_by_id('login')
      username.send_keys('my_username')
      password.send_keys('my_password')
      login_button.click()
      print("Login successful!")
      driver.quit()
      return "Done"