input: Write a python function to extract all HTML tags from a given string.
  assert extract_html_tags('<html><head><title>Test</title></head><body><h1>Hello, World!</h1></body></html>') == ['<html>', '<head>', '<title>', '</title>', '</head>', '<body>', '<h1>', '</h1>', '</body>', '</html>']
  assert extract_html_tags('<div class="container"><p>Sample text</p><br/></div>') == ['<div class="container">', '<p>', '</p>', '<br/>', '</div>']
  assert extract_html_tags('<ul><li>Item 1</li><li>Item 2</li></ul>') == ['<ul>', '<li>', '</li>', '<li>', '</li>', '</ul>']
output: def extract_html_tags(s):
      import re
      return re.findall(r'<[^>]+>', s)