input: Write a python function to send emails using the smtplib library.
  assert send_email(['test1@gmail.com'])== 'Email sent successfully.'
  assert send_email(['test2@gmail.com', 'test3@gmail.com'])== 'Email sent successfully.'
  assert send_email([])== 'No recipients provided.'
output: def send_email(recipients):
      import smtplib
      from email.mime.text import MIMEText
  
      if not recipients:
          return 'No recipients provided.'
  
      # Define email parameters
      sender = 'your_email@gmail.com'
      subject = 'Test Email'
      body = 'This is a test email.'
  
      # Create the email
      msg = MIMEText(body)
      msg['Subject'] = subject
      msg['From'] = sender
      msg['To'] = ', '.join(recipients)
  
      try:
          # Set up the SMTP server
          with smtplib.SMTP('smtp.gmail.com', 587) as server:
              server.starttls()
              server.login(sender, 'your_password')
  
              # Send the email
              server.sendmail(sender, recipients, msg.as_string())
          
          return 'Email sent successfully.'
      except Exception as e:
          return str(e)