input: Write a python function to navigate to a URL, click a link, and take a screenshot using Selenium.
  assert navigate_and_screenshot("https://www.example.com", '//a[@href="link"]', "example.png") == "Screenshot taken and saved as example.png"
  assert navigate_and_screenshot("https://www.test.com", '//a[@href="test_link"]', "test.png") == "Screenshot taken and saved as test.png"
output: def navigate_and_screenshot(url, link_xpath, screenshot_filename):
      from selenium import webdriver
      from selenium.webdriver.common.by import By
      import time
  
      # Initialize the WebDriver (assuming using Chrome)
      driver = webdriver.Chrome()
  
      try:
          # Navigate to the URL
          driver.get(url)
  
          # Find the link using its XPath and click it
          link = driver.find_element(By.XPATH, link_xpath)
          link.click()
          
          # Wait for the new page to load
          time.sleep(2)
  
          # Take a screenshot and save it
          driver.save_screenshot(screenshot_filename)
  
          return f"Screenshot taken and saved as {screenshot_filename}"
  
      finally:
          # Close the WebDriver
          driver.quit()