Hi Charles,
One thing to note is that you probably don't want test transactions running on a live page, doing so would mean that anyone could use a test card on something meant to be live and not in test. You'd likely want some sort of toggle on each donation page that your user can set to determine if it is in Live or Test Mode. Based on that toggle, in your database, you'd determine whether or not to use a Live or a Test API key.
Test cards only work with a test API key.
I believe you're using Stripe Connect. If you're using the Stripe-Account header, then switching to test mode is just a case of swapping your platform's live API key for the test API key. Alternatively, if you're using the Access Token method, you can use the user's refresh token to obtain a test API key while leaving the live API key active. You'd set the appropriate key based on that Live / Test toggle in your database.
Matt