input: Write a python function to scrape a webpage and save the text of all 'li' elements to a CSV file using BeautifulSoup and the requests module.
  ```python
  assert scrape_and_save('https://example.com', 'scraped.csv') == True
output: import requests
  from bs4 import BeautifulSoup
  import csv
  
  def scrape_and_save(url, filename):
      response = requests.get(url)
      if response.status_code != 200:
          return False
  
      soup = BeautifulSoup(response.content, 'html.parser')
      li_elements = soup.find_all('li')
  
      with open(filename, 'w', newline='') as csvfile:
          csvwriter = csv.writer(csvfile)
          for li in li_elements:
              csvwriter.writerow([li.get_text()])
  
      return True