How do you place the form in test mode?

How do you place the form in test mode?

Here are some ways to disable your form or limit the submissions:

How-to-Enable-Disable-a-Form
How-to-Limit-Submissions-in-a-Form

If you want to test the payment integration, then make sure that it is in Sandbox mode. Kindly check these guide:

How-to-Test-your-PayPal-Payment-Form
How-to-Test-Authorize-net-Payment-Tool-on-Sandbox-Mode

Hope that helps. Thank you.