input: Write a python function to fetch all users from a SQLite database using Flask and return it as a JSON response.
  assert len(fetch_all_users()) > 0
  assert isinstance(fetch_all_users(), list)
output: import sqlite3
  from flask import Flask, jsonify
  
  app = Flask(__name__)
  
  def fetch_all_users():
      conn = sqlite3.connect('database.db')
      cursor = conn.cursor()
      cursor.execute("SELECT * FROM users")
      users = cursor.fetchall()
      conn.close()
      return users
  
  @app.route('/users', methods=['GET'])
  def get_users():
      users = fetch_all_users()
      return jsonify(users)
  
  if __name__ == '__main__':
      app.run()